How to get my puppy to rest

This may seem like common sense but I am quite emotional with worry right now!

My 4.5mth old puppy has strained a muscle in his back leg making walking around painful. He is generally VERY active. 2 x 30min walks a day and racing around the yard.

I need to keep him resting for the next 3ish days to recover. How do I do this? I feel horrible!

We have been to the vet.