The point still remains, though: if Bob chooses $5 now instead of $500 a week from now, he demonstrates that he prefers the $5 now to $500 a week from now.
That's YOUR assumption. He may be demonstrating that he isn't aware of the $500 option. He may be demonstrating that he isn't aware that $500 in a weeks time is better than $5 now. He may be demonstrating that he doesn't know that 500 is a bigger number than 5. Or many other things that mean he's not making an informed choice.
Of course these sound unlikely, but that's because you chose a hypothetical, rather than a real case. When you go for real scenarios, the alternative explanations are also more real. As I pointed out in another post, people take out loans at 3200% APR. They are almost certainly not making an informed choice when they do so.
But going back to a real example, similar to yours. They've done experiments with kids. Eat one marshmallow now, or you can have 2 marshmallows if you resist eating for 3 minutes. Very young children can't even consider the 3 minutes away option. they eat the marshmallow straight away. A bit older and they try to wait 3 minutes, but fail, and temptation cause them to eat the single marshmallow. Older still, and they develop the ability to patiently wait for a bigger payoff.
In your example, choosing $5 now rather than $500 in a week may be just a sign of immaturity. An inability to envisage future benefits as outweighing instant gratification.
Who are we to tell him he's wrong? We are the people with the knowledge, wisdom or maturity to understand the deal. There is no great good being served by leaving people to make decisions that are bad for them, purely because they lack the intelligence or maturity to make better decisions themselves. You may think there is, but that would be just a religious belief in freedom of the individual being the greatest good. It's not even common practice in the USA. For example with seat-belt laws we accept that the good to the person and society of wearing a seat belt outweighs a drivers freedom to make a stupid decision.