A lot of what I have to say has already been mentioned before but I just wanted to add my opinion.
First, I’d check with his caretakers as to the type and amount of grain he’s getting. If he’s laid up, then he definitely doesn’t need as much grain anymore and it could be contributing to his excess energy.
Irritability could also be caused by ulcers. I’d talk to a vet, but I’ve heard that sometimes it’s easier to just treat them for ulcers than try to diagnose them.
In addition, a lot of Thoroughbreds (but not all) need someone with quite a bit of experience to handle them on a day to day basis. I’ve seen a lot that try to take advantage of you if you’re too soft and get scared if you’re too firm. A lot of bigger barns have people clean stalls that aren’t really “horse people” so mishandling of him by others when you’re not there might be creating an attitude. I would try to find out who is cleaning his stall, feeding, or otherwise handling him on a daily basis and just watch how they interact with him or other horses. Do they look like they know what they’re doing? Are they unnecessarily rough or do they get walked over? A lot of people think behavioral problems come from people being too rough (and I can’t stand to see people get mean with a horse and it does cause problems) but I’ve seen more bad behaviors come out of horses being handled by inexperienced (or just ignorant) people who let the horse be the boss, especially spooky and pushy (sounds more like what you’re describing)
It also might just be that he’s fed up with being cooped up and you just have to wait it out. I have a mare that is a perfect angel and can teach kids how to ride on her if she’s turned out 24/7 (even if she’s not ridden in a while) but if she’s kept in a stall for more than a couple hours, she’s quite a handful for even an experienced rider/handler. How much longer of a recovery does he have?