Skye,

First I would make sure that his teeth are in good shape and have been floated with in the last 1-2 years. This way no matter what bit you use, you will get the best response possible. A horse can ride all of its life in a snaffle with no problem and you will plenty of control. A snaffle can be a very gentle bit or a very harsh bit depending on how the rider uses his/her hands. I have had some horses that prefer the standard snaffle and some that absolutely refuse to work in one. My mare rides in a D or egg butt snaffle, my 30 year old gelding rides in a Tomb thumb style, I ride another horse that rides in a mid port, with a roller. I prefer to find a bit that the horse responds the best in with the minimal amount of pressure. Their are some great books out there about what each bit does. Mylar has a book that explains each different bit and how they work.

You could always put a snaffle bit in his mouth with out the reins attatched and put your hackamore over the top and let him get use to it while you ride. You could also just put a snaffle on him and let him wear it in the round pen while you work him, biting him up lightly so he gets use to how it works on his mouth.

Hope this helps, be safe