Baby first cold (question)tmi

This cold is in his stomach or chest. He is wheezing but when he has phlegm in chest. The phlegm in his chest is literally going down into his stomach and every time he eats or is basically awake he poops. He’s pooping maybe 3 to 4 times an hour liquid hot poo. He’s so uncomfortable and is screaming because it has burnt his butt.

I’m just looking for two recommendation what would you do for the phlegm and how would you treat his bum? I’ll probably call the doc on Monday but I don’t think they’ll be able to do much.

I’ve applied Desitin and it provides temporary relief. He’s 11 weeks old.