If he's 8.5, he's only a little on the young side for third class, isn't he? So he's not madly young for his class now.
the biggest problem I see with moving him up again is what age he'll be coming out of secondary, and starting work or college. At that rate, even assuming transition year, if he skips another year he'll be only 17, which is definitely on the young side. And if he doesn't do transition year, he could be starting college at 16, which is way young, imnsho