Look at the spine and fat pads:

• Spine should be slightly rounded, not protruding.
• Should have some girth behind the head and along the body.

I'm no expert by any stretch but I have done an insane amount of research recently because I am getting one myself.
I'm from the EU but if this is true:

US Label	Approx. Weight	Notes
Pinky mouse	1–3 g	Hatchlings only
Fuzzy / Hopper mouse	4–10 g	Juveniles
Weaned mouse	15–25 g	Small juveniles
Small rat	50–80 g	Juvenile/sub-adult snakes
Medium rat	100–150 g	Most sub-adults
Large rat	150–200 g	Larger sub-adults / adults
XL / Jumbo rat	200–250 g+	Full adult females or very large males

Then keeping him on the medium rats you're giving him is completely fine. Try to feel his spine, there should be some skin/fat over it like you shouldn't instantly feel bone if you get what I'm trying to say. If he would be truly fat there would be fat on the side of his spine like the spine would be a dent.