Thanks for taking Cupid into your home! He's sure to heal better there than in the shelter. Lab puppies are the very definition of energy in motion (we've raised 4 of our Labs from puppies) so not running around is against his very nature. Personally, I'd recommend talking to the vet about possibly using something stronger than herbal Rescue Remedy if he is moving around too much and not letting his leg heal (e.g. which is the more important thing right now so he has many years of running/playing ahead of him). I've used Ace (Acepromazine) with both my dogs and horses when it's important to "take the edge off" -- in small doses, I've never heard of long term side effects and it's cheap.
On the more productive side, this is a great time to be working on his mental training which helps with his boredom. Clicker training with simple tricks that don't require back leg movement (watch me, take it / leave it, shake, and maybe some light tug/drop) would be perfect to teach this pup who probably hasn't had much training.