You can talk to your vet about it, and acupuncture might be able to release some of the tension. Pole exercises and lots of stretch time should help, but whatever you do, consistent upkeep will be key.
I use a chiropractor who does acupuncture as well and that helps adjust the problem but if you don't take care of the muscles then he/she will just get stiff again in a couple weeks. I make sure that my horse gets out 4-5 times a week for a ride and I do hills to build his butt muscles up to give him support. He gets stretched before and after every ride as well as getting a liniment massaged into his muscles when he is done his workout. I have had good luck with Mineral Ice and Sore No More. Also, turnout is key to keeping the muscles loose. My horse is out for at least 12 hours a day.