I have a stifle stretch that very well could help him when you can't ride him! I also have a few more tricks up my sleeve. PM me if ya want?
I do his stifle stretches whenever I am out there. The vet showed me. One is just basically holding the leg back (as if you were a farrier) and the other is holding the leg forward and up toward the belly (also stretches the hock with the stifle).
.....although I didn't do his stretches today when I buzzed out there at lunch because I didn't exactly want to get my khakis dirty.
You got any other stretches besides that?
I should do some other more often (and I'm going to make myself) but I also stretch his front legs forward, stretch the shoulder (by letting him relax the leg and rest his toe, and not sure of the name of the stretch but I call it his front end yoga pose, because you pick up a front foot and bend it toward the opposite knee.
I got showed all those last year when I had Red get a massage. She gave me a few other pointers too, besides the stretches.