The right hasn't fully fused. There's still joint space in there. Even on the left, the intertarsals are fully fused, but the tarophalangeal is still open. The might never fuse further, or maybe they will. Horses can be sound with terrible hocks, and lame on clean hocks, so if he's not showing any issues, then just be aware and keep an eye on it. I'd be interested in knowing why he's so fused at a young age, it'll help you manage it in the future. It'll be different if it's from an injury vs just stress of being a horse. You may consider starting something like Adequan or Pentosan. If it's any solace, his hocks look better than my horse's at his age.
I'm not familiar enough with navicular xrays to comment on it.
The thing about xrays is when looking for small changes, it's easy to either miss them entirely or confuse something benign. A mm of rotation the wrong way or the beam at the wrong angle completely changes what you see.