!!!!!UPDATE!!!!!
We kept our cage of adult mice next to Mr. Slithers's cage overnight and then took him with us to a local pet store this morning and they put him in a tub with a fuzzy. He didn't seem interested for the first few minutes but then we picked it up and dangled it in front of him before we were going to put him back in his travel cage to leave, and he ended up striking at it and ate it! He has had no trouble with water since we've got him, and we try to monitor the heat/humidity in our snakes' enclosures very closely. Being new snake owners we are trying our best to make sure we get things right.
I am extremely happy that we finally got him to eat! I am hoping that we are on a road to recovery for the little guy, as we were extremely worried about him! I know everywhere we read even the hatchlings didn't weigh as little as he does. We were thinking pinkies were too small, but weren't 100% sure; because of how thin he is it was hard to tell. I texted the guy and told him he needed to take better care of his snakes and that he needed to be feeding them bigger prey than that.
I called animal control but because he wouldn't give us his address and had us meet him somewhere instead they said that they couldn't do anything.I feel bad for the guy's snakes. He is really shady. When we first texted him with our concern he started talking about how he's had tweekers like us want our money back after buying snakes from him because they needed the money for more important things and so on even though we never once even said anything about wanting our money back. That was also his excuse for not wanting us to come to his house, because he's supposedly sold snakes to a tweeker at his house and then ended up coming home one day with his house broken into and cleaned out. He's just a really shady person and I wish that we could somehow get him caught. He had told us that the snake is two months old and that he was feeding them pinkies weekly but I don't know if I can believe that. I think he is neglecting them and trying to sell them as younger than what they are to people who don't know much about them since they are so thin.
Anyway, I'm glad he is eating and hoping that next week he will eat on schedule with our normal and pinstripe ball pythons.
On another note, our normal, Luci, shed her skin today after being in shed for a while! All of them like their prey live, and we are graduating our two females up to rat pups next week as we are sure they are ready for it. We don't have a name for our pinstripe yet as we just got her a couple days ago from a nice lady who is getting interested in three-gene pythons.
ALSO, after doing a bunch of research, we think Mr. Slithers might be a yellowbelly, because he has that beautiful vivid color and his stomach isn't white, but instead yellow. I don't know if that could just be a sign of him being unhealthy but I have read that yellowbellies can be easily mistaken for normals.
Thank you everybody for all of the advice and being so kind, I feel like I chose the correct place to ask questions and happy that nobody tried making us feel crappy about anything because of being new owners. I will continue to update occasionally on how Mr. Slithers is doing for anybody who is curious about his recovery, and if I have anymore questions I will definitely be on here asking them!