OP - After watching and rewatching the three videos several times and doing quite a bit of research (as well as referring to some past experience), I would tentatively and unprofessionally suggest your boy has developed arthritis in his hocks. Because of this, assuming this is the case, his pelvis and/or SI and/or stifles can be out of sorts (from the swiveling motion in his hocks placing extra stress on muscles, throwing things out of alignment). This can also explain a front end "lameness"; ie over-compensating. As well, it could explain him getting up oddly.
The great news is that if this is the case, treatment options are very optimistic and pretty cheap. A vet and chiro are your first solutions, calling for radiographs to determine degeneration and fusing of the joints (lower joint fusion = good, upper = not so good), as well as adjusting the pelvis and/or back/etc to help realign things. He may or may not (if this is all true) require injections in the joints, though personally I am not entirely for them. Lastly, physio exercises and semi-regular chiro visits to strengthen the muscles to hold his joints in place.
I have a friend whose 10 year old Saddlebred jumper developed arthritis (mild) and it caused all sorts if issues. After a few months she was already back to normal, and now over a year later, she is in a regular chiro schedule and still jumping regularly.
Fear not! There is always hope.
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Last edited by RunSlideStop; 05-24-2012 at 08:43 PM.