A king snake that's a year & a half old needs to be eating much more than pinkies! Are you talking about pinkie mice or rats? He should be eating hopper size mice or small adult mice- yikes, he's just hungry!
But since he's been underfed for so long, make a gradual transition to make sure he can digest okay- you don't want him barfing up his meal due to insufficient digestive enzymes- which requires good nutrition (enough proteins etc) to produce. So start with fuzzy mice- several per meal- then after a few meals, upsize again to a hopper. Feed weekly. No wonder he tried to eat your arm, poor snake is starving. He might have smelled your BP on you too- king snakes eat other snakes, along with the typical rodents etc.
He could use some more "socializing" too once he's better fed-but please do work on feeding him first- pinkies have very little nutrition. The more mature rodents offer more minerals (bigger bones) as well as more protein. So don't push too much activity until he gets more food in him- he needs to build up strength.
Usually snakes don't just pee "water"- their urates are usually whitish & chalky (soft & moist but not liquid, though there can be some liquid too) but if that's what he's been doing, it's because the pinkies he's been eating are inadequate & mostly water.
King snakes normally out-grow being fed pinkies in a few months- as they grow big enough to eat fuzzies, that's what they're fed, & so on up. Any similar king snakes I've raised have been eating adult mice by the time they're 1.5 years old or 3' long.