Don - might also give some thought to "preventing" him from 2-up riding on his bike, at least until some time passes and he has experience riding. Also, he might prefer the more aggressive look of a solo rider than a brat.
Obviously, he would have the bike for a very long time, so 2-up would become practical, but hopefully by then, he'll have a stable full from which to choose. This was my approach with my boys, and I have no regrets. Besides, he can always change the seat later.
My $0.05 worth (which isn't usually worth a penny to most).