Very red and angry groin

Hey OH bought lo's nappys the other day and they are a bit big for him. He didn't think it would make a difference but after a couple hrs sleep in one he's got a red angry rash on his groin that he wants to itch. I use sudo on his bum every change but when put on the groin it drys his skin out. So I tried putting vaselin on it which doesn't dry it but doesn't help? Any tips!