I agree about him being hot just because he hasn’t been worked in a long time.
Is there any way he can get turned out longer? This would be the safest way (for both of you) to help calm him down and burn off some excess energy before starting back. Being out even if just walking around, not running, and having social time with other horses does a lot for their minds.
I would advise against lunging because small circles put extra stress on the legs, which is not a good thing for a horse coming back from a tendon injury. If he is well behaved enough on the ground to not be yanking at your bad hand (chain over the nose is a good idea) then hand walking will help for sure. If you can, walk him where you’ll be riding him.
When you get on, if he’s been pretty good on the lead rope, it might be a good idea to have someone walk beside him at his head or even lead him for the first couple times.
You can also talk to your vet about giving him a mild sedative the first few times you get on. If he’s good, lower the dose the next time, if he’s still good, lower it again til he doesn’t need it. Not everyone is keen on the idea of sedating their horse but honestly sometimes it is the safest way to rehabilitate a hot horse. You don’t want him bouncing around on that leg and risk straining it or re-injuring it.
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