Mastiff Puppy Limping...?

Hey, I've got a mastiff puppie- 12 months old and he seems to be limping. I know his leg isn't broken because he wouldn't walk on it in that case but I don't know what's wrong it's been weeks. I don't want to take him to the vet because it's not cheap and all they can do is give advice... What's wrong with him? What can I do