Stifle injuries can take months to heal. Did the vet ultrasound or suggest sweating his stifle to tighten it back up?
We had a mare that had stifle problems and we sweat her hocks with DMSO and Furasone for the first week to help tighten it up. We put her out in the indoor arena on level ground daily (all day) so she wouldnt build up any excess energy and for the first several weeks she was hand walked on level ground in a straight line (no circles or backing until some strength and stability was gained).
Her entire healing process took us 5 month and we did not get on her back to ride her until then. She was ridden lightly first at a walk for a few weeks, then jogged and now finally after about 6 months she is back to her normal self.
It certainly depends on the injury and how severe it is. It is better to air on the side of caution if you want your horse to be sound again and stay that way.
Backing is a great exercise to do to strengthen the stifle but our vet did not allow us to back her until she was very sound in her stifle again. Again I believe it depends on your vet and what advice he/she gives. Make sure your vet is good with lameness issues also. Best of Luck. Take your time and it will heal.