This is something originally developed by a boy so most likely it evolved over the years to include multiple methods. It may use shorthand/abbreviations, bad grammer, mis-spelling, and slang. A single cipher method is highly unlikely.
It's probably simple in that it will have one step between plaintext and ciphertext, and words are not transposed. Also it looks like numerics are not encoded
Brackets or a line around a section could seperate trips, days, people he met, conversations on the phone, or anything, but we can probably safely assume they're in chronological order.
That 71,74,75 sequence could be revealing. Could they be house numbers, was he a train-spotter, is it a long term plan for his old age?
And could "NCBE" mean "No Change Behind Eddies" referring to a fruitless search for change dropped by drunks behind Eddies Bar or something?
What this job needs is detailed knowledge of his life; people, places, interests, habits etc. because without that background there's no way to determine what he's referring to. If all such data were dumped into a database to make a word index, that could help (maybe not so useful I just thought it up).
But FBI, if background info can't be released, at least make large size scans of those notes available in TIFF format (>12bit) so people have a chance to see what the letters are in the noisy parts.