Ooh sounds like a lazy horse! It's fab that he's so.. comfortable with you riding and not uptight, but at the same obviously it's great for a little enthusiasm - you might want to have a look at a different bridle/bit, because apparently with a little more control you might be able to get him going. Also, try and find something that he really enjoys doing, not that he doesn't enjoy riding normally - perhaps he's bored? Vary what you're doing and you may find he perks up, i had the same problem with my old loan horse. She was lazy because she knew the routes and didn't feel the need to be alert. Maybe try new things he's not used to - so he has to be well alert, you might find change kickstarts him into moving - hope i helped xo