I must have the pickiest Cali King out there. High white, low food drive, LOL. He finally ate two mouse hoppers last night after fasting for a few weeks due to being in shed and he's never been big on eating rat pinkies or fuzzies, though occasionally he'll take one if I start with a mouse fuzzy first to get him into "food mode". He's not a soaker though. Have you checked for mites? Did he soak a lot with his prior owner?

Agree you should check his temps, mine is in a 40-breeder with a hot and cool site. Cool is mid-70's, UTH with t-stat gives a hot spot of 90*F.

He also looks small for a four year old King.