His loan officer told him his credit score would reflect more positively if he used only about 60% of his available credit line each month, and left 15 or 20 dollars per month in carryover balance, instead of paying off the entire balance each month.
Truth or bullshit?
Bullshit, to an extent. First, FICO only sees your balance as it's reported. When your card company reports it to the credit reporting agencies depends on the card, but it'll typically be shortly after the statement date. You can use 100% of the card's credit, then pay it off a dozen times over the course of the month but if the issuer reports $0 to the CRAs, the FICO score pulled will reflect that.
You also want to be careful about where you leave balances. Now this is one of the areas where different FICO formulas will give different results. The overall message is this: having a zero balance reported on all cards is fine. You'll have pretty much full points on the revolving credit portion of your FICO score by doing this. It's possible to squeeze a small number (typically single digits) out of your score by keeping about 9% utilization on your revolving accounts. Where it gets messy is that different scoring models treat where that balance sits in different ways, but I honestly wouldn't worry about that; just pay them off and make sure they're at $0 when the issuers report them. You can find more about this sort of thing at the MyFICO forums at http://ficoforums.myfico.com/
Keep in mind that if your son is looking for a vehicle loan, that's going to be an auto-enhanced FICO score, which you can't get your hands on until he actually applies. That won't change much from what you can pull on your own if he hasn't had car loans before. As a general rule, if he's had car loans and paid on time as agreed, his auto-enhanced FICO score will be somewhat higher than his regular FICO score. If he's had car loans and didn't pay as agreed, it can be a bit lower (possibly quite a bit lower).
Word of advice on vehicle loans: shop around and be smart about it. Did you know that if the dealership shops you around to different banks and finds one that'll finance you at 5%, they can tell you 10% and keep the difference? Know your score before you walk in the door so they can't try pulling a fast one on you. Also, don't walk in the door without financing already available. Why? Because you then have all the power and you can shop like a cash buyer. Since you have a relationship with a credit union, it's likely you've been thinking about just going through them for it. They probably even have pretty good rates (credit unions are often quite good). When you apply for some types of credit, like a credit card, the issuer will do a "hard pull" on your credit to check your score and credit history and that inquiry will cost you a few points. However, some credit types like mortgages and vehicle loans have a grace period allowing you to shop around for the price of a single inquiry. So when he's ready to buy, have him make sure his report is accurate and clean as possible (no 30 day lates or other delinquencies on any of the three reports - and yes, check all 3), then shop around with the credit union, other banks you may have a relationship with, anyone who's advertising good rates, etc.
Many places will run through a quick approval process over the phone and if approved, they'll mail you a check good for x number of days (usually anywhere from a couple weeks to a month or two) for up to y amount of money. The loan doesn't actually happen until you fill out the check and hand it to someone, so start with finding the best deal before you go to any dealership, then get that check in hand. Obviously shop around for the car as well, but once you've got it narrowed down to a few dealerships, walk in and let them know right away you have that check in hand. Negotiate like you've got a suitcase full of cash in hand because that's essentially what you have. Try and find the dealer invoice price before going in there and start there (they'll have rebates and such above and beyond that, so don't let them fool you into thinking that's what they'll actually pay for the car). Further, if you're getting something that isn't a super hot seller, let them know you know they're paying every single day to floorplan that car and make it clear you'll walk if they won't meet your price. I've literally been asked to leave a car dealership before for low-balling them only to get a call back later that day agreeing to the price or getting extremely close to it.
Once you're set on price, you get to play a fun game. See, you've got that check in your hand which is at a certain interest rate (let's say 5%). Now the dealer wants you to use his financing because he can make more money on there that way, especially since you just pounded him on the price. So what you tell him is if he can beat your financing (and give yourself a little room, tell him it's at 4.5% and that's what he has to beat), you'll let him finance it. If he can't do it, you sign your check (he will). Where you want to be careful is manufacturer financing where they tease you with 0% or 1%, something to that effect. You almost always have to give up rebates and other incentives to get those rates, so when they tell you they can do your financing at 0.9%, ask specifically what you're losing to get that rate. When they tell you it's the $2,000 cash back offer they'd figured into the original price (and this is where they're real assholes and honestly should be prosecuted for fraud because they wouldn't have offered that tidbit of info - just put the new figure on the financing forms and waited to see if you noticed it), you're going to need to do some math to figure out the best move. I recommend using a vehicle loan interest calculator (PSECU has a good one here: https://www.psecu.com/vehicles...).
Doing all this, I've literally walked into a dealership with no money in hand (just a draft check from a credit union), bought a car at ~30% off MSRP, and walked out with 100% financing (as in I didn't hand them a penny or write any check or let them touch a credit card - just signed my name) at well under 2% with full rebates and incentives intact. At that rate, over the course of the five year loan, when adjusting for inflation, the bank paid me about $300 for the privilege of financing my vehicle and I paid less in inflation-adjusted dollars than someone who saved up all the money to go in and pay cash. I hate negotiating (and I'm terrible at it), so I simply hand them a piece of paper with their dealer invoice price, taxes, tags, rebates, destination charges, etc already calculated and a final number of what I'm willing to pay and tell them to please just let me know if they can meet that number. If they won't, there are plenty of other dealerships and somebody always plays ball.