and for the last month I have gotten serious about cavaletti- 5 days a week. Then he had 3 days off and when I came out yesterday, he kept "stumbling" with his hind legs on the lunge again.
It's possible he's just weak, particularly in the stifle, from habitually running around in his inverted, trailing posture. If he started to improve with proper exercise and then reverted with time off, that's a hint to you, isn't it? hills, cavaletti, lunge in side reins, ride him in a way that encourages him to step under and round up. Cheaper than futile vet visits- if he seems happy, and improves, great; if not, then go spend a fortune for a good workup.