Sorry to read about this. From your description, I would have thought he'd ingested some gravel. It's possible it may have gone further back / been swallowed, and he may try to pass it out the other end.
What are your readings for ammonia, nitrite and nitrates, using a liquid-based test kit? Although you haven't changed your tank maintenance regime, it's possible that the water supply maintenance has altered a bit, which can sometimes lead to alterations in water quality.
Does your water company use chloramine and, if so, does your water conditioner address that?