Props to you for taking the little guy in and trying to get him set up properly. What are your temps now, and what are you using to measure them? A 20gal is fairly big for a young BP, and sometimes all that open space can make them feel insecure. I would suggest either covering three sides or adding a bunch of fake vines or crumpled paper (less attractive, but economical) to make the enclosure feel more secure.
Also, leave him alone for a few days before trying to feed, since he has a history of refusing. Feed in his enclosure. If you are going to try f/t first, you can try the "zombie dance" but if that doesn't work try leaving the f/t prey item in overnight. Some BPs are shy eaters. If he doesn't take f/t, you may want to wait at least 5 days and try a live prey item.
I would also try something bigger than a mouse fuzzy. A hopper or small mouse would be a better choice. My hatchlings take mouse hoppers for their first feeding, so I guarantee your guy can eat something bigger than that.