This sums it up really, my son was riding a 20″ when he was seven (about the same height). He was still riding at 8/9 and could have ridden it for a bit longer but the components started falling to bits. When he moved to 24″ he was unable to do a lot of the things he could do on the 20″(wheelie, manual, jump etc.) It was just a bit big and it pissed him off, luckily he had a bmx to keep him interested. If your son likes messing around in the woods, riding gravity stuff and getting air, a 20″ would be much better.
That’s a fair point, ok to go bigger if your not doing anything particularly awkward. We tend to do mainly flat trail riding with the odd tight turn here and there so the bigger bike is turning out to be ok. I suspect had we been doing more faster down hill bumpy stuff maybe the smaller 20″ would have been more managable.
We went to 2 different bike shops and had 2 different answers, one said get him the 20″ the other said get the 24″
It comes down to your lads ability and where your likely to be going I suppose.