My kid's 50's are stock except for the body lifts and the handlebar lifts. I have also taken out the baffle and sparkarrester and moved the needle down one notch. What else can I do for say free or under $50 ?
Like already mentioned, a good set of tires is always a good idea. The stock enduro-like tires are almost dangerous if he's riding at any speed. You can get those for roughly $40, than a Uni pod filter and $11-15 depending on flavor and reseller. With the new filter and baffle removed, you probably want to go to a 60 main jet and keep the needle raised, or possibly a 62 if you have good air where you're at.
Some other cheap mods you might consider are a folding shifter ($10-35), a 15 tooth front sprocket ($8-15), skidplate ($40-80), machined flywheel ($75), good footpegs ($60-90), and tons more. Depending on his weight and riding style, you may want to improve the suspension so he can better use the power he already has. Lotsa options and ways you can go with the sky being the limit. You can also check into used parts and find some bargains from time to time.