Is sending him to a rehab facility an option? I’ve going through literally the same thing - a stifle ligament tear - and my horse just would. not. settle. at the barn where I had her, despite trazodone and ace. I moved her to a rehab facility out of desperation and it’s worked beautifully. Everyone there is on stall rest so everyone is just standing around, its nice and quiet, minimal activity, plus they are pros at handling and medicating hot horses on rest. It’s worked out so well they’ve actually been able to get mine completely off the drugs (for now, at least).
I feel your pain so much. The only other solution I can think of is more drugs. But it sounds like he’s at the roiling ball of nerves stage which is hard to sedate.