I second having a vet/chiro examine him. My horse was tracking the same (except it was LH that was swinging in and under.) After a good chiro adjustment and some accupuncture, he was tracking normally.
My mare does this. It's a combination of a lateral-medial imbalance of her hind hooves, and SI issues. I'm working with a farrier and a DVM/chiro to correct this.
I would start from the hoof up. Have a competent farrier evaluate the medial/lateral balance of the hind feet. Once that checks out, get him evaluated by a vet/chiro. If the feet are unbalanced the chiro work won't hold. Get the feet balanced first (if it's an issue) then get a chiro eval. and adjustment. I recommend using a vet who is also a chiro.
Something to consider before using a chiro is to have any muscle spasms released. If they aren't released prior to the chiro then the adjustment won't hold. A good body worker or massage therapist can help you with that and, perhaps, even eliminate the need for a chiro . And yes, have hooves done first, also -- again, if hooves are imbalanced the adjustment won't hold well.