I thought my new Foxtrotter didn't go well in any type of bit with a broken mouthpiece after I bought her, because she was always chewing and mouthing it. She was also sort of nervous in general, a bit barn sour, and would kind of get antsy whenever I gave her leg pressure (like to side-pass).
I rode her all summer (except when she was near foaling) and now I find I can ride her in a snaffle or tom thumb, no problem. She just needed a lot of riding and getting to know me (and me getting to know her). She side-passes like a charm now too. So in my mare's case it was anxiety. The anxiety of a new horse in a new home with a new rider.
So I can't speak for your guy, but it was that way for my girl. I also try to give her loose rein unless she is absolutely trying to go forward when I don't want her to, or at a speed faster than I want her to, then I check and release and/or circle around bushes and trees. But I think horses are always happier with less rein contact. Or at least only light rein contact.