Are you sure you don't kick him (accidently, of course) when you get on? Any health issues with back or something? Did he do it before or just started recently for no reason?
I'd recommend to keep using helper for a while. Ask not to hold reins, but hold the long lead rope. So pony is "kinda free", but the helper still can stop him if he bolts. So pony will figure out that he'll be stopped in any case.