Doesn't look unhealthy.. Looks proportioned. If its easier to feed mice (for whatever reason) you can give him 2 every 7 days instead of 1. Also different stores call Rats different names at diff sizes. Make sure to keep something with you so you know the correct size to get him. Sometimes once your at the store what looks small enough will end up looking Huge compared to your snake once you get home. Hahahaha. At least you cant go wrong with a mouse size at this point. He can eat a Jumbo so they cant screw that up.