My sons have been riding since they were 3.5 and 4yrs old. There is a HUGE difference between a 85 and 125. What does he ride now and how tall is he? For him to be safe and so both of you have fun I would find him something that fits his size and riding capabilty.
85cc MX bikes are light, offer terrific suspension and brakes, but the engines are much less forgiving for trail riding. 125cc MX bikes are similar but are as tall as a full size bike and typically weigh 200 to 220 lbs after. GG made a EC 125 that has nicer manners with some low end and midrange but they are a little hard to find.
A really good option for an in between bike is the KX100. Yes it took me a while to get over the color but it was the only real option in that size for a light weight bike with good suspension and brakes. The KX100 engine is based upon the KX85 engine and it shares much of the port sizes so it is less peaky with the extra 15ccs. It has decent low end and good midrange. The KX100s make great transition bikes from smaller 4 stroke bikes toward big bikes.
They are also more fun for fathers to ride than 85cc bikes ;-)
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