I owned the horse Kstinson mentioned in her post. I had the same issues with my big gelding.
Boe got progressively worse over a year. He was very healthy, sound. But always tripped on his forehand, and went down many times, even in the arena.
I would get a vet to rule out wobbles. All the research I did said it isn't very common, but that it happens to taller male horses more often. Boe was 16.2. They have several tests they can do, like pulling on the tail to see how the horse reacts.
I'm sure he is just being lazy, but after my experience, I would rule it out. It can be dangerous for both of you.