Ball python with poor appetite, not growing

I bought a male banana ball python from a reptile expo in Denver, CO. When I got him on 8/18/2015, he was 99.2grams (3.5oz). In that entire time, I have only gotten him to feed around 5 times, and he has put on 5.6grams (.2oz). He now ways 3.7oz. I have tried live rats, f/t mice and rats, different sized prey, a different tub, different lighting, feeding him at night, feeding him during the day, warming him up before hand, adjusting his enclosure temperature/humidity and changing substrate. He hasn't eaten in two weeks, and he's half the size (at least) that he should be. I have gotten him to strike at prey, but when he misses, he gives up.

I'm at a loss for what to do because, while he is still active, and still sheds, he isn't gaining weight. Nothing about him is abnormal except his appetite.

Have you tried live mice? Probably near the size of a weaned mouse.

How do you feed him? Do you watch him eat or do you just throw in the live mouse and walk away? I have one female who will not eat if I am there. Weird, but I think her attention is ore focused on what I am going to do more then what the mouse is going to do. Have you tried ASFs. African Softed Furred Rats are native prey for them. Are you feeding in his enclosure or a separate feeding bin? Feed in his enclosure, feeding in a separate bin is stressful and you will only run into more problems.

I have tried both live and f/t, feeding him with tongs, or just leaving it in the cage.