Hire him. That's all you need to do to make your "I did not disagree" part in good faith. Put him under the watch of a supervisor of some sort and have that person monitor his performance. Your other employees will be watching. You can't afford to lose morale or profits and he needs to know this. So does the rest of the family.
BTW, valued employees often go with the business when it's sold. I have to wonder if he is cutting it at the current job. If not, you've got some problems ahead, but careful planing a strategy can make it all work out for everyone.
The nephew may not like the world of construction. I know I HATE what it has become. In the old days it was long blond hair, a rigging axe for a framing hammer and a surfboard tied to the ladder rack.
Now, it's just a lot of nasty work with nasty people ****ing over everyone else on the job. Not that is anything really new.
Oh, and if he doesn't speak Spanish, he will be next to useless in the field. I'd put him in sales on commission.
(after 6 mos. of site work, just so he knows what he's selling).