Responses
It could be because of the bad homemade pudding. (My initial guess)
Or because he ate that whole bag of pudding.
I don't think the author thought the refrigeration thing through, but then again, it could be the reason.
You shouldn't leave custard type puddings out because they're mostly dairy and eggs which spoil quickly if they're not refrigerated.
The toilet scene... was it because they ate pudding that hadn't been in the fridge? I kept thinking if it could be eaten if not stored cold. http://www.mangago.me/read-manga/nii_chan_icchatte/an/nii-chan-icchatte--chapter-2.html/35/