How tall? I'm 5'4", 145. An 85 is way too small for me. A 100 is about the perfect size for me, but I wanted more power, so I got a 125, cut the seat foam and had the suspension lowered.
I rode the bike for awhile without cutting the seat foam or lowering the suspension. I could ride it fine, tall though it was. However, if he's around my height he will need 3-6" tall starting blocks so he can touch the ground with both feet on a 125 2 stroke or 250 4 stroke.
You don't want to be dragging your feet on the ground on an MX bike. Tiptoe with both feet is about right IMO.
BTW, when a bike is properly set up for your weight, the rear suspension is supposed to "sag" down about 4 inches lower than the total suspension "travel" when the bike is sitting on a stand with the rear wheel off the ground.
Thus, when properly set up for him, the 125/250f may be lower than they seem in the shop.
As noted, you can lower the seat height by removing the seat cover and cutting the foam down a couple inches. Then you can replace the seat foam when he gets taller.
The 125 and 250f likely have more power than he's used to, but if he's reasonably sensible, I'd think he could spend a week or two getting used to the power spread and then he'd be fine. Lots of kids his size/age have done it before.
That should get you started.