Just go at it slow. I put my arm over their head, with my forearm between their ears, with the headstall hanging from that hand. Get him used to that, and with the bit in your hand, use your thumb to get his mouth open.... if he doesn't want to open with just the thumb, set the bit on his lower lip - don't touch the teeth if you can help it. And use one, two, three, or all 4 fingers at the corner of his mouth to get him to open, then ease it in. You can perhaps practice getting him to open up without the headstall at all, too.
Around here, there are a lot of half assed 'trainers' that smack the bit on their front teeth, and try to force their mouth open by pulling the headstall so the bit pushes on the teeth to force them to open. It's got to hurt, and when they've been banged enough, they fight. I'll see if I can get some video to show what I mean.
Ask Often, Demand Nothing, and Reward Generously.