Is my 18month old baby allergic to something in his bath water?

My son has always had sensitive skin around his bottom and I have had to change his brand of nappy and wipes before in the past, as well as his baby bath and moisturiser. It's never the whole area of his bottom just patches and it feels dry and a bit scaly to touch. It's red and feels slightly raised. He has been to the doctors before and has been given hydrocortisone cream which did eventually take the patches away but this took about a week. But even when he hasn't had the red patches on his bottom, when he gets in his bath after been in for a few minutes patches appear and he starts to really scratch leaving really red marks. When he gets out of the bath the itching and patches disappear after about 20 minutes to half an hour. I don't use anything in the bath. Can anyone help?