Often, the first thing that is recommended on a "new-to-you" boat is to be certain that you can register it in your (or your son's) name. It would be a shame to put a lot of work in, and they be stuck because your neighbor never properly transferred the boat from where-ever he got it.
From that point on, it's just a case of figuring out what you want from the boat. Mostly, the best thing to do is to be certain that it is safe, and then just go fishing a number of times. After you and he have a half dozen trips under your belt, you will probably know what has to be changed.
I can't tell you how many modifications I made on my earlier boats, only to find out that what I did wasn't what I later wanted. That's why I screw together any PVC project, instead of gluing it. After I figure out if the item does the job, then, I glue it together. The same thing applies to working on "improving" your boat, IMHO.
Ther are tons of really well done "build threads" on this site. Read, measure (twice), and then cut.
Have fun, richg99