I'd ask you bet but if its muscle related I'd try sore no more it might help :)
I had thw same problem. It turned out that he had a back pain. Id recommend consulting the vet first.
Talk to your vet right away
have you ever gotten his hocks injected? my horse use to get it but then we stopped for about two years and he got really stiff again for a while until we got them injected again
my friend's horses back legs have that happen too. When she rides him she does a lot of stretching at the beginning to loosen him up and makes him work through it a little bit because sometimes it's just from him sitting in his stall and she gives him cosequin in his am feed which helps with his joints and makes him move better. But if it looks bad she gives him 1 gram of beut at night and he should be 10x better the next day