Hello! I am new to this whole thing and have been trying to read as many of these threads as I can. I purchased a male BP at a reptile convention on July 28th. I was told his hatch date was January 22, 2018, so he was 6 months old. I had a cage all ready for him, which I have since learned is much too big (40 gallon). I left him alone to acclimate for a week, and then tried feeding him. I put him in a small plastic tub with a thawed hopper mouse. He ate it after a couple minutes, no problem. Fast forward to the next week, I did the same thing and he ate again, no problem. That was his last meal. I tried again each week for 6 weeks to no avail. I tried different times of day, I tried leaving him in with the mouse for an hour, we didn't handle him 48 or so hours before trying to feed him, I made sure his water was full and fresh, his tank heat and humidity was consistent... So this last attempt to feed him (number 7 I guess), I tried something different. I put the little plastic tub I'd been feeding him in, into his tank on the middle of the temp gradient. I thought this might make him feel more safe, and I could leave it in there for a longer period of time, and inside the tub, the mouse wouldn't be sitting on top of the substrate (I took it out and warmed it with a hair dryer every 3ish hours). Y'all, I thought I broke him. He stared at that damn mouse without moving for 12 HOURS. Finally I took it out to see if it would break him out of his spell. He crawled up in his tree and didn't move for another 12 hours. Thinking I must have completely traumatized him, I covered his cage a little more than halfway with a towel so he couldn't see anyone moving about the house, and have not disturbed him other than to peak in and refresh his water. It took a couple days, but now he is moving about like he used to.

Important notes:
Right after attempt #7, I thought to weigh him on our food scale for the first time. He was only 106 grams. I do not know how much he weighed when I bought him.
To my very unprofessional eye, he seems like a healthy looking snake, just small. He doesn't look triangle shaped, he explores his cage actively, doesn't seem nervous, slithers all around when people hold him, I see him drink water, and he pooped before attempt 7 and two days ago. He has never shed with me, and he doesn't look dull like he is about to shed either.
His cage is a 40 gallon tank with a mesh top. He has 3 hides and plants he likes to go under. He has a big water dish on the cool side, and a small one one the warm side. He has a climbing stick and a climbing tree. His humidity is always around 55. The cool side stays just above 70 and the warm side is just at 90. He used to love his small hide on the warm side but in the last 2 weeks has stopped using it. I found a piece of poop inside it, removed it, and have since deep cleaned, but he still doesn't use it. Now his favorite spot is in the leaves on top of his tree. The tree is on the warm side under the ceramic heating lamp. There is also a heating pad on the warm side. I use coconut fiber substrate.

TL/DR: 8 mo. BP only weighs 106g, hasn't eaten in a month and a half.