Weird question


So this might sound like a dumb question butttttt how do you tell if your newborn is allergic to a diaper brand? My little guy is 2 weeks old and has diaper rash. He has had it since he was about a week or so old. For the last couple days I have noticed that it seems like his whole area that is covered by the diaper is red. I brought it up to my husband and he doesn’t think it’s an allergic reaction but I don’t know what else it could be? Our first son wasn’t red like this that I remember anyway.