My 1 year old has rash on face and mucus in stool and poops every 30mins to an hour For month now. we have taken him off milk but he still has it. We left our house for 3days to go to grandmas and it was gone tge next day. Then two days after we came home it came back.
If you can repeat your experiment again and things clear up, then you know he's reacting to something in the home.  If that is the case, it is less likely that a specific food is the cause, but rather a chemical that he's being exposed to or some other environmental irritant.  It would be highly worthwhile to repeat the stay away, and see what happens.  Keep all other variables the same while away (i.e. don't change diet, clothing type, or other things)