Baby BPs are a bit goofy sometimes. And sometimes they just don't know any better. One of mine hatched in July is still not eating on her own and another only started eating a couple weeks ago.
At 90 grams he's eaten a few meals already. So he has done it. I think you may be stressing him out more by helicoptering. The immediate jump to IBD is not usually made when it comes to a 2 months old in a new environment. As far as the Neuro symptoms go, does he have a gene in the spider complex? Was he overheated or exposed to chemicals at some point?
Did you wiggle the mouse a bit after he coiled? Or was it maybe a bit too cool by the last strike? He may have just realized that it wasn't 'alive'.
A baby that young very little experience so he only knows a few things about food. Did you ask the breeder what he was eating? You could probably afford to wait it out another 5 days and try again with f/t or try a live of the breeder was feeding that. It's not very hard to switch them to f/t after a couple of good meals, but the stress of the move and new home may be a factor there.
Speaking of which.... What is he set up in and current temps?
Baby BPs are a bit lanky, and a lot of older BPs have a lot of chonk. So it's most likely the case here, but a couple photos of the little one wouldn't hurt to get a body condition opinion.