I wouldn't consider him big for his age by any means. I have some 9 month old balls that are easily 3 times his size judging from your pic. He appears to be a little thin as well. He really should be on appropriate sized rats at his size. Small-medium mice are far too small for him. At the least he should be eating large adult mice.
Other than that, I'll second what the others have said about your temps and getting a decent thermostat. Whether you go for the VE200, Herpstat, Helix or similar you will be happy. They are all great thermostats.