Dunno if Kings have significantly different body structure than corns, but I'm guessing not too much different.
- Can you make out the ridgeline of it's spine down the length of his back? If so, may be skinny.
- Can you see his ribs? If so, definitely skinny.
- Is his skin 'baggy'? Yes? Skinny.
- Does his belly suck in concave, or is it nice and flat across the bottom? Former - could be skinny; latter, probably not.
- When you hold him, how does he feel? Mushy without much substance? Skinny. Firm and muscular? Not.
You just got him. You just started a feeding schedule with him. Even if he is a bit skinny, you're on the path to getting him growing. Fret not.