Your his Dad, so you care about the whole picture.  I say don't worry too much how he will put his children through college and those type things.  College isn't for everyone as it has been discussed here many times. He and his wife will have a priority of their own there.  Like others have said, get an accountant and all the real numbers.  If you and your bother want your money with interest over ten or 20 years, maybe he would be just fine.   As long as there is cash flow.

If he is really competent and has that fire, this will probably be his only chance at this dream, so I would let him decide if the risk is worth it.  I am 40 and have 3 small children.  I am in a different situation then your son, but if I were in his shoes, I would make it happen.  I would take over for dad and uncle and make it work.  1000 times!!!