It's not a good sign when tropheus stop eating without an apparent reason like aggression during feeding time. The fact that he's noticeably less active suggests that he needs to be treated.
Tropheus and empty stomach will lead to bloat. Tropheus have one of the worst digestive tracts in that they can suffer very easily from blockage and on an empty stomach and a weakened immune system, can succumb to bacterial/parasitic infestations ie. bloat very easily.
This typically happens in colonies and sometimes the entire colony falls ill without the owner even noticing. Part of the problem is that tropheus are notorious for mouthing feces from other fish and it spreads quite rapidly in that fashion.
If it's a case of mere blockage, which does occur in adults but not so often in youth, he'll more than likely pull through with some TLC. If it's bacterial/parasitic, then clout or metro are the only way to go.
If there's no change in disposition within 48 hours of quarantining, I'd strongly advise treating him.