I have my adult Mexican Black Kingsnake in a 32 qt tub and he does just fine in it. Temps are a hot spot of 85 and cool side is 75-73. From what I've heard, Cali kings are similar as far as care requirements. I'd think he should be eating something a little bigger at that age, but it depends on his size. A lot of pets stores and uneducated owners seem to underfeed their snakes, so if he is small for his age his meals should be smaller too.

And as far as prey species go, I've never given my kingsnakes rats. Just mice all their lives, pinkies for the baby snakes and small to medium adult mice for my adult MBK. No need to give them rats when mice are the perfect size.