Wanna slow him down?....take off those training wheels. The T-wheels are the problem because he is really riding a motorized three wheeler that goes and goes...seriously you wanna be careful letting a 3 y.o ride a bike like that with those training wheels. Most of the PW50's I have seen with t-wheels had ones similiar to that of a regular bike. Your T-wheels extend out quite a ways which create more stability but also act as something that can hit a tree, rock etc and send him flying.
I started my boys out at 5-6 using a Honda Z50 with NO training wheels. It is important for the kids to gain respect for the machine they ride and that includes tipping, spills, crashes etc. Bikes act much different when only two wheels are on the ground. I doubt he is to that point to cruise on only the two wheels and if he does, might be too young to figure out when trouble is near and might end up hurting himself.
My boys are all riding (12 y.o on a TTR125, 10 y.o on a YZ85 and my youngest 9 y.o is riding a XR80). It takes time and practice to get them to the point that they actually feel comfortable and safe riding the bike.
3 years old is pretty young especially on that PW50 because that is a two stroker that moves out pretty good. You could always install a larger rear sprocket to slow him down even more.