Hi-well my first suspicion is that your horse is being mistreated somehow by the staff at the new barn…I have had my mare at many farms and when she developed bad behaviors it was due to mistreatment (she would throw her head for example at a place that they hit her in the face-NEVER an acceptable form of punishment!)
if thats not the case, then i agree with pulling him off grain/pelletd feed (I NEVER recommend pelleted feeds for any horse, oats or corn only for calories and then choose a multivite supplement for nutrition–THE best way to be sure your animal gets the right amount of nutrients and calories since they are no longer intertwined based on feed dosage. oats are my fave cause corn is fatty and unless a horse isnt holding weight, i recommend oats.), and also just grass hay in addition for calories.
He will also need special attention while he is in rehab-special attention should be paid to ground manners during this time, like etiquette in the crossties (remember that it is a process, not an event! keep calm). He may enjoy a nice grooming! My mare has been on stall rest on several occasions (she is a VERY expensive horse with all that has happened to her!)-and i find some relaxing music can be HUGELY helpful with her attitude. There is special relaxation music out there, but any classical music will be helpful as well. heck, even pop music is better than nothing (i DONT recommend anything like death metal or anything loud and noisy though). I would allways bring a boom box and CDs in order to listen with her while i was there.
as well-does he have a toy to play with? my mare mostly ignores her jolly ball-her fave toy in the world is empty plastic milk jugs (she seems to like the gallon ones better than the smaller ones), with the cap removed and hung from the bars of her stall with some bailing twine (one length! never make a circle/loop with it or a hoof or head could get stuck in there!) I give it a foot or two of slack so she can bat it around and horses love the crinkly sound that it makes! They need to be changed when they get beat up enough (read flattened), but they are essentially free so i am happy to give her them!
you can also ask your vet what activities ARE allowed-like can you take him for walks on his lead line? If you are nervous about him spooking, do the walk INSIDE a fenced area like an arena or pasture-that way if he pulls loose, he can run too far.
since you are here at smartpak, can i suggest considering a calming supplement? if your horse is on other supplements, please be sure not to overdose him on any one thing (selenium for example, could be found in more than one supplement type). You can consult with your vet about recommended amounts. I have my mare on Vision, and it has really helped her anxiety from being visually impaired due to uveitis.
which brings up the point, how is his vision? eye problems can make a horse spooky…since they cant see predators as well. sometimes injuries can exacerbate eye problems, like uveitis.
if he will be on lay-up for a while, i could suggest some essential oils that could help with anxiety as well, but they would need to be ordered online and by the time you got them, he may be done with his layup by then. just let me know if you want that info!