I an rule out CO2 poisoning because everyone else exposed would also be ill.
Yes, of course he may have swallowed a non food or toxic item. Things that are fine for us can kill a cat like Tylenol. chocolate, grapes, raisins, onions. Furnace oil or antifreeze can also poison him.
There is also the possibility he had an illness when you got him that was misdiagnosed or not treated properly.
He may have had a genetic abnoormality.
The only way you would know for sure is with a necropsy.
As long as the other pets are eating well and eliminating normally and have no symptoms of illness, this is not contagious.
I would suspect whatever he had when you got him made him ill and was never fully resolved.
Once again, I am so very sorry for your loss. Your children must be very upset as well.
Please let me know if you wish to discuss this further and I will be here for you.