There is a lot unusual about his biology.
The most obvious thing is that his heart has at some point been replaced with a crystal of similar size. It moves the blood around it via magic in a smooth heartbeat-less way. There are also diamond cabochons embedded under his skin in hard-to-notice locations. Presumably those are also doing something, though they're not active at the moment. A kangaroo-like pouch holding various objects is hidden in his torso.
Beyond that, he's a shapeshifter who has optimized his body for survival and convenience in hundreds of ways through the centuries. Stronger bones, stronger skin. The spine redesigned to be less terrible. Eyes which can see into infrared, ultraviolet, and the planet's magnetic field. Changes to the neck which make him harder to decapitate or strangle while still retaining most of the neck's natural motion range. Entire systems, like the sinus cavities, look like they've been taken apart and rebuilt intelligently at some point.
There are a number of things that possibly look worrying, but are presumably intentional given the rest. Nothing that is dangerous to him, or to anyone around him, at least. He is as healthy as it is possible to be.