Soemtimes I resort to yanking and I REALLY DON'T WANT TO DO THAT!
He has no respect for the bit.. Running with a snaffle or anything mild doesn't mean you are keeping the horse light. Just the opposite.
If the horse doesn't respect the bit you have a heavy horse.
Would it hurt to borrow a western curb bit with a chain and give it a try???
I am not saying you have to use one but try it, see if it works.. If what you have is not working then experiment with borrowed bits until you get something that works for you.
A strong bit used softly works alot better then a mild bit used harshly.
Have soft hands but when the time comes to really set the boy down use everything you have an pull him right on his butt. Teach a little respect and you will have a much lighter horse on the bit.