How can I get my horse to give to his bit?

I have a Morgan horse, he is about 18 or 19 years of age. When i got him he had 90 days training in Montana (the only training he has had). When i got him he was still a little green, now that i have had him a year this June, he has gotten much better under the saddle. Now here is our dilemma, he came back from Montana using a snaffle, it was to weak for him. He has a very soft mouth though, so I bought him a level 1 Myler Swivel Shank. He learned to bend with it (which is good) but he is still very head strong. Should I buy a more aggressive bit? I would LOVE to use a hackamore on him, but I am to afraid he'll take off on me. Any suggestions would be lovely. Thank you for your help. :)