An option you can consider is going to a curb bit. A good curb bit used properly is not harsh...in fact, I'm beginning to think one used properly is gentler than a snaffle. There is a sticky thread by smrobs about curb bits: Bit Information (Curb and Western type bits and hackamores)
If a horse grabs a curb bit in his teeth, it doesn't help him any. Snaffle bits work in a straight line. Curb bits rotate, and gabbing the fulcrum in his teeth doesn't change the pressure. They also provide plenty of warning, so the horse gets to choose if any pressure is needed at all.
This video was done by one of the forum members. I think it has a lot of good information on transitioning a horse to a curb bit: https://www.youtube.com/watch?v=TTyM...TOE2D5kF7OxdRA
If the problem comes when he feels too separated from another horse on the trail, work on it gradually - 10 feet, then 15, then 20, etc. Every few rides, increase the distance by 5 feet and let him get used to it gradually.