LogFAQs > #934712201

Just have him sit on the toilet. If he goes give him tons of praise and maybe some candy or something.
If he refuses don't reinforce any behaviors like crying or whining. And give bigger and better praise for going.

Start slow though, just have him sit bare ass on the toilet for a bit. Work up to actively peeing. If he does tell you he has to pee, even if it's in the pull up, have him produce in the bathroom. You want the act of voiding to be associated with that room and the toilet.

Source: I'm a behavioral therapist. I literally potty train Autistic 3 year old's.