What is good for dogs to build up strength?

Today I found a stray dog and the vet says he needs more mucle mass.

Try going to the beach or lake and see if the dog likes swimming, this helps build up muscle mass. Or if the dogs well enough try going for runs or go to a open space like the dog park so the dog can run around not only that but socialize! If the dog is very weak try starting with light walks then maybe every week go for longer ones and so on Hope i helped!!!

excersise you should've also asked the vet

A dog physiotherapist will be able to give you some good strength building exercises for you to do at home together with your dog. You can do various incline exercises and resistance exercises.You can also try taking your dog to a dog pool (heated) for a swim and walk him with a weight pack. You need to start off with the right amount though. The physio will be able to give you a program to work to.I used a dog physio after my dog had acl surgery last year and after a few weeks her bad leg ended up being even stronger than her good leg so I know it works. The exercises from the physio are super easy and only take a few minutes a day. I would hope your vet has a contact for a dog physio, otherwise just jump online and find one in your area.