Have you signed up for puppy kindergarten classes yet? I HIGHLY recommend signing up as soon as possible, as he'll be ready after 12 weeks... you should look for classes that are positive reinforcement only, and that include or offer puppy play classes, too. Puppy class is so important not only for training basic commands with distractions, but also for socialization with people and other puppies.

It sounds like you're doing fine with your new pup. I put a harness on Fozzie when he was little, too, and he definitely prefered that. I would never allow the puppy off leash in your backyard. You can buy a really long leash (30+ feet) at any pet store for very cheap... just let him drag that around and step on it if/when he tries to explore outside of your yard. It's never too early to start practicing "come", too! Make sure that every time you say "come", you run the other way, act excited, and reward him with lots of treats and praise when he comes to you, and practice as often as possible.