Most small ATV's have throttle limiting screws to keep them at a reasonable pace. I don't know what the exact speed is, but I know some of the plastic ones do 5 MPH. You might be able to hit that , but I doubt it'll choke down to 2.5. You have to keep it revving enough to engage the clutch. Even if there is no limiter screw you can cut a small piece of PVC and put it in the carb to keep the slide from going all the way up to reduce top speed (like with the PW 50 dirtbikes). My son had a Kawi 50 ATV that wasn't that fast with the limiter in. I'm sure someone here has one you can put him on to test it out. Hope that helps.
Thanks for the reply!! It does help. I know he will be happy with whatever I get him I just want it to grow with him. So I dont have to keep buying more and more LOL yea right