Helping lamenes issue

I have a TWH/Morgan. I had noticed that his back rear hoof seems "twisted". This has gotten progressively worse over time. My farrier told me that is how my horse walks.
Yesterday I was observing him in the pasture and notice that when he puts that hoof down his hock appears to move outwards and back in (really the whole leg does but since the hock is in the middle thatis the motion I see.)

I assume this is a development issue for muscles. He is 5 this year.

What would you call this gait issue?

Also, yesterday when I was riding him at a walk adn had him pick up speed to go into a faster walk, several times during the one hour ride he had a "sinking" sensation in the rear. What is this called? Suggestions on how to help him?

Thanks

I would consider this strictly a lameness issue. I recommend you discuss his condition with a veterinarian. If he has a sinking sensation in the back while you are riding him, I would not ride him until I had a vet's approval. It sounds like he is having a difficult time carrying you.

Good luck!