Getting rid of the 40 gallon tank would most likely solve his eating problems. Babies his size I keep in 6-15qt tubs, and have gotten every single one to eat a few days after they arrive.

Get a 10 gallon or a 15qt-27qt tub, set it up, put him in, wait a week, and then drop a small adult mouse in. It's highly likely that he would eat without a problem.