Ok. I’ve never changed the diaper brand in 1.5 years. My son now developed this “rash” across his…

Ok. I’ve never changed the diaper brand in 1.5 years. My son now developed this “rash” across his entire backside today. It’s been 2 hours diaperless and it’s not going away. Allergic or chemical reaction? What do you do for that? Sorry the picture is a weird color. Tried to show how red it is but it’s not showing up well