Are you sure it's the bit? Since you said you're pretty green yourself.
Do you have riding lessons or a trainer you could ask? It could be that you aren't giving him any clear aids (riding with his mouth) and he's becoming resistant or he simply doesn't respect the bit and can pull the reins out of your hands or gape his mouth and evade.
I'm not sure what's going on because I'm not there. Or it could simply be he's being a poop head and you need to guard your reins and not let him pull through.
Curious to see what other HF members say..
"Strength is the ability to use a muscle without tension"