In what ways does he act generally unhealthy? How is his weight, appetite, fur quality (a poorly bun often looks dishevelled, the fur has no sheen to it) pooping and peeing habits? Has anything in his appearance or behavior changed?

Not all buns do much. Some are quite content just existing in the same room as their human. Does he hide all the time? Can he relax “out in the open” a cpl yards from you? Many rabbits can be ambivalent towards close contact with their human. They both want it (it’s nice being pet) but they’re not quite sure if it’s completely safe. So they can growl or lunge and then immediately demand pets or a treat after. Fearfulness is to an extent hereditary, as is aggression.

It is a bit weird though, since you’ve had him since he was so young. Do you know anything about his circumstances before you got him? I had a male bun who’d growl and box, then demand pets. He had been living feral, it kind of showed in his behavior, even though he cant have been more than 5-6 months max when I got him.