Could he have eaten something too hard and big to digest? A rock, a marble, a snail, some gravel...? I just tried to do some research... It seems a few people have had similar issues and every thread I looked at was basically people just saying that they're sorry and had no idea what could have definitively been the cause, and they all seemed to pass away.
I'd suggest putting him in the fridge, daily water changes... I'm sorry I couldn't be more help. Good luck.