I wouldn't change what he's eating if it's overweight. Just the period between feedings. Going from rats to mice cute certainly be the issuse. As could then going to a pup.
I would go back to what it was eating and go from there.
Either way 4 weeks is nothing for a healthy snake.... Even more so for an overweight snake. Might even be best. That of course it's just going of of what your said since I'm not there.