Tripping can be caused by several things: physical reasons, the horse is lacking motivation and is seen as "lazy," rider error, or the horse is so unconfident that he's just trying to get away and he's not thinking, he's just reacting.
If you can rule out any physical issues, and check yourself to make sure you aren't dumping your weight onto his forehand, then I would guess it's a motivational issue. He probably doesn't see any point in doing what he's being asked to do...it's boring to him so he doesn't really try, hence the stumbling. So mix things up for him! Put down ground poles, go on a trail ride, do a lot of Freestyle riding (loose reins, working toward bridleless riding), more straight lines than circles, over jumps, up and down hills, transitions, all kinds of things to mess with his brain! Also do a lot of point to point exercises to get him to WANT to go forward.