Weight pulling is an excellent sport to build muscle! You can start out now by getting a harness and just getting the puppy used to a light drag and begin understanding the whole concept of pulling something behind him. You will have to wait until he is older before you start adding some serious weight and it wont be until he is older that he starts to fill out and really gain his muscle. I am getting into WP right now with one of my dogs. There are plenty of WP boards out there with some awesome advice. If you want the link to the one I go to please PM me.
I would also look into getting a spring pole or a flirt pole ( for DOGS) LOL These are great for excercising when you cant go for a walk or what not and bully breeds especially seem to love them.
As mentioned above, backpacks are awesome! Again though, you will have to wait until your pup is older before putting much weight into it. I didnt put any weight into OC's backpack until he was 10months old. Before that I would just put the backpack on him and fill it with plastic bags so he would get used to having the extra bulk on his sides and the heaviest thing I would put in there was my wallet (with no change) and then a balancer on the other side to keep the weight even. Just now I am using the backpack for every walk we go on that is 1hr or less (unless its a hike and the dogs are carrying supplies). Currently Ronan carrie 4Lbs and OC carries 5Lbs. It may not sound like much but you gotta remember that a dogs weight is much less than ours so what seems like something very light to us could be quite heavy for them. You should NEVER exceed 1/3 of your dogs total body weight. Also make sure to never start at or near that maximum weight, build up SLOWLY. Same with weight pulling. Start slow and work your way up or else you could very seriously injure your dog!
Swimming is another great excercise! If possible, have your dog walk/swim against the current for an even better work out.
Another major part of putting on muscle is diet. If you feed crap food you will get crap results, simple as that. I feed a mixture of raw meat and kibble (Orijen and Innova Evo) along with some other supplements.
Also remember that if your dog is neutered, you can expect him to gain muscle like intact dogs. Dont compare him to some stud a breeder has on their kennel website, just strive to reach his own personal best! Good luck and have fun! Never push your dog to do something he doesnt want to do! It should be enjoyable for the both of you.
