I haven't had to fatten up a dog but I'm wondering if a grain-inclusive food would work better for him than a grain-free. It sounds like he gets plenty of calories, way more than mine do! Maybe this coming week, when he's on house arrest for his leg injury, he might pick up a few lbs.
The Rimadyl might make him feel so good that he will want to run and play the way he normally does and not give the injury a chance to heal. I'm not sure what instructions the vet gave you but I do some serious rest if there's an injury- only go outside on a leash and, if necessary, crate them or confine them in one room inside the house. Having some Kongs frozen with kibble mixed with peanut butter in them might both keep him busy and add a few calories.
Good luck, keeping a lab quiet is a challenge!