exactly as Dixie said, you can also try a treadmill but I would wait until he is more conformable to introduce that. The flirt pole is something you can easily make, if you don't have one. A spring pole can be made easily as well. If your back is good, get a nice rope toy or Katie S Bumpers Sq2 Super Sqwuggie Fire Hose Bumpers Squeak and Tug
and let him tug and pull to build up the muscles. Make sure NEVER to leave the rope toy around he should not have the chance to chew/eat the rope, just use it when tugging.
Welcome, nice to see people from Mass (I'm in Quincy) good for you for rescuing him! Please post a picture as well if you are not familiar with the breed he maybe normal. A lot of pit bulls are overfed and I was surprised when I saw what a healthy weight was at first.
Also, just because he is not aggressive with other dogs does NOT mean it will never come to the forefront. Be sure NEVER to leave your dog with any other animal without you being there 100% of the time. and please do not EVER go to Dog Parks.