Hi, it's your truck and you can do what you wish. However what I have seen is that a kid seems interested, but once you get into the project (normally the nasty parts like sandblasting) they loose interest. You will then have one truck apart, while the other one you could drive around is gone. $3500 will not go very far in restoration these days even for parts. If it would be worth say 10,000 that might be another story. It probably would be better for him if you can work on one and you can drive the other one with him.
Maybe some other project could bring in $3500 which he could do, or a well meaning relative could loan him the money. As the other guys have said, if you have a truck in the family it's a shame to see it go, almost everyone I have known regrets it later.