This isn't about training. This is about rehab. It is about controlling a large animal so he doesn't hurt himself or you.
I've rehabbed more horses than I care to count. I'm rehabbing two right now. Until they are allowed turn out and are back in normal work I will not hesitate one second to use chemical assistance to allow us to have the best chance at a full recovery.
Eta: swimming (or an underwater tredmill) is great especially for stifles but not something I would do until further into the rehab. Make sure you talk with your vet on that too.
That said, most did learn a few things in rehab (even using drugs) but since you are typically limited in turning and lateral work during rehab, training will be limited drugs or no drugs.