Is he in shoes? It really sounds like it could be hoof related because you said he trots sound on grass. How about uphill or downhill? If he has heel pain, he will be more lame going downhill. If he's barefoot, have you tried hoof boots? Sorry, but there are so many possibilities.
Tried more turnout. It actually kinda backfired. He is a bit goofy, started kicking and racing around.. wiped out all together. Got up okay after that, but we've been putting him into a smaller enclosure to try to discourage too much silliness. He doesn't appear to be lame when he is trotting on his own on the grass, but he also does not do it for long so it' kind of hard to observe.
The vet ruled out hoof issues, but did encourage the ferrier to trim the angle on the front toes more. So he's been doing that, as instructed.
Also did some blood work to rule out some kind of systemic problem, just in case!