Definitely invest in obedience training as soon as you can. It's a great thing and I just started last night..... It's literally the very first thing we're working on in my class. In the meantime, keep him on the leash and stand in front of him. Move backwards quickly calling his name until he comes and then reward him immediately with praise and a treat. Movement is the key when calling him. Repeat with him until he comes willingly each time.
Be a responsible pet owner, Spay and Neuter.