My old guy is a long story, and there are many factors that led to his arthritis and his retirement, but the rotation of his left hind was sort of the "last straw". Yes, he is on supplements-Cortaflex w/HLA and MSM. THey help some. He is also out 24/7, which helps keep him moving.
He was actually so badly cow hocked at birth that the vet recommended he be PTS, but the breeder rigged something up with PVC pipes, etc and straightened him out enough so that he gave us many many years of sound riding. He is also a draft cross, and was started jumping too early....which has not helped matters. I bought him 18 yrs ago as a 4 yr old and he was jumping then.
I did not know then how detrimental it is at such a young age.
Good luck-it is tough for sure.