6 Month Black Lab

I have a question about a black lab I have from rescue that almost died. From what I know he almost died from heatstroke due to the fact he was confined outside and was struggling to get free. When I got him he seemed very thin, and weak on his legs, vet at center said he is ok just needs time. What is a good method of helping him gain leg strenghth some healthy weight? He is playful and I keep food an water always out. He has been to the vet and the vet says he is fne and the weight stenghth will come with time.
Another concern is he does not sit right, he sits on the side of his hindquarters and he has a difficult time getting up and down. Is this becaue he may have muscle strain in these areas? Will his muscles heal? According to the vet he is fine and he will get better. Is there anything I can do to help him heal and gain healthy weight quickly? Any info would be great so I can get him heathy, he is a beautiful lab. Thanks