I'm pretty much a beginner myself, I've had my thayeri for about a month now but I've done my homework about these snakes. So I'll give my two cents since no one else has, and then someone else can check on it later.
How long has he not eaten for? From what I understand kingsnakes (especially males around breeding season, which is still around now somewhat) sometimes go on hunger strikes as do all snakes and will refuse to eat. It depends a lot on the snake I think, some are easy feeders and some just aren't. They'll probably more readily take live mice and some like deer mice over lab mice, so you might want to try that if he continues to not eat. Also, how hot have you heated the mouse up, and with what did you heat it? (ie - microwaved mice are often ignored, or so I've heard). And if he's two feet long then he sounds like he might be able go on to fuzzies by now, mine is a foot and about 3 inches and eating one pinky a week.
I'd tend to think that the size of the cage won't effect his eating that much, especially since he's not an adult yet so a 10 gallon won't be incredibly tiny on him. Hopefully what I said helps, and someone with a little more experience can fill in anywhere I might have missed. Good luck with your new kingsnake, hope he'll work up an appetite soon!
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-Danielle
0.1 Thayeri Cross - Arella 
0.3 Cats - Cupcake, Smokey, Thelma
0.0.1 African Dwarf Frog - Kikia
1.0.3 Tropical Fish