'Lead walks only' - oh, I remember that, with an active dog!
You'll have to tire him mentally, since you can't physically - show him lots of new things, do lots of little training exercises - he'll love it, and it'll tire him nearly as well as running about.
When I had my JD in the same situation, it was late summer and the blackberries were out. I showed him how to pick blackberries and we sauntered along the hedgerows, picking and munching the blackberries as we went.
When he was back to fully fit, he showed the other dogs the trick, so then we'd be all 5 of us grazing the hedgerows on walks!