My child is on Nutramigen formula, has been since 4 weeks old . The past week she has had what you’ve described, pooing during every feed, with mucous present (and some red bumps on her genitals). I’ve tried to think back to my first born and I believe this is a delayed reaction to the 8 week jabs. It occurred two weeks to the day, from when she had them and the rotavirus liquid. Could your son’s nappies be a reaction to the 12 week jabs?
My little one had this alongside vomitting we tried for 7 weeks multiple different formulas for cmpa and they all made him 10x worse we’ve gone back to normal cows milk today he has had runny poo but thinking that’s just because we straight swapped him. I’d be careful and if there’s no other symptoms it’s likely not to be cmpa we’ve been through 7 weeks of hell and him not sleeping on his back thanks to the cmpa formulas when we don’t actually think he needs it😢
It could be still due to a stomach bug or could be down to teething Although it could be an allergy I'd personally wait a few days to see if it resolves if not go back and demand to trial a cmpa formula for 6 weeks