Make home not so pleasant. He may be looking for excuses to head home. Take him only until you feel him begin to tense then turn and walk back but when you get there, work his tail off in the yard, driveway, whatever. Circles with lots of change of direction. Really push his butt around. When he's puffing hard you'll feel him want to quit. That is when you start riding him away again. You may not get far but just do as before. It often takes 2 or 3 times before a horse catches on that he gets to relax at the walk, and breathe, when he's leaving but has to work like hell at home. If you do this every time you trail ride, as a refresher, he'll quit looking for excuses to go home.