Potty training trouble

I've been potty training my 3 year old son for the past few days and it's going well. When he's naked he will go on the potty, with no issues. Since he's doing well I put underwear on him and explained how to pull them down to use the bathroom. But so far we've been through 3 pairs today, he pees right in them. When I offer to help him pull them down he just runs away and won't let me. What should I do? I don't want to put a diaper on him when I go somewhere but he just pees in underwear