For what it's worth, I'd go for the 24" on the grounds that the lighter a bike is the more kids enjoy riding them. There's also the fact that a heavy adult entry level steel mtb may not fetch that much when the time comes to sell, whereas parents looking for a lighter bike that will fit their kid are usually prepared to spend a bit more.
We've had, over the last 20 years our Saturday club has been running, any number of parents who've gone the lighter route after they've tried the "grow into it over longer" strategy.. There's also the factor of component size. While a lot of kids bikes are appalling in that respect, decent brands usually put some thought into it, so he'll find it easier, and therefore safer to use, especially the braking side of it, assuming that the adult bike has longer reach levers, of course.
Anyway, good luck - I hope he enjoys it whatever decision you make.