| I would decide which length of rein gives you a good contact to his mouth, without pulling constantly, and without giving him too much freedom, and hold. Don't pull, just don't give in to him either. And if you have the arm strength to just hold your ground with him, he's really just fighting himself. Don't change anything in your arms or your riding, and just hold your ground, eventually he'll realize that you aren't fighting him back, and it's a losing battle, and should just settle into it.
Just make sure, not to pull at all. That you are holding him at a good length so that he shouldn't have a reason to pull.. with just a light contact. So that you know YOU aren't causing the problem it is him. It may take awhile, and he will fight you for a bit, but eventually he will realize he can't win against someone who is not fighting him. It's like if you're in an argument with someone, and you're screaming, and they just say nothing... eventually you stop screaming too... because eventually you run out of points to argue.
The only problem is that, this being a horse that you only ride once a week, or less, will still be continuing these bad habits with other riders... so you will probably have this same argument everytime you get on him for the first part of your ride. Good luck! | 
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