[Jim727]

A promise is a promise. Done deal.

However (there's always one of those) you did not promise to keep him regardless of his productivity. I would hire him, but give him the "no special privileges, carry your load" lecture. Failing to make him earn his place on the crew would be a disservice to the kid. Have him report to a supervisor other than yourself and give said supervisor the same lecture. If he works out, you have done well both by your brother and him; if you lay him off for non-productivity you have kept your word to your brother and provided the kid with a needed life lesson.