How do i get my dog to gain muscle?

I have an 8 month old bull mastiff and i want to start building up muscle on him. should i start now or should i wait until he is older? what should i start with (please don't say to just walk him everyday because i already know that). i want some advice on proper training to make his muscles stand out. i also want it to be safe so he doesn't injure him self. thank you!!!