Constant Diaper Rash!

My son is a year old and since he was 6 months old, he’s had a constant diaper rash! It’s not on his butt or thighs, it’s on his waistline. We switched to hypoallergenic diapers/soaps, but he has to constantly have diaper rash cream on or he breaks out on his waistline. He is perfectly fine until someone forgets to put the triple paste on him. Then he’s all red and inflamed within hours. I’ve asked his pediatrician about it on multiple occasions, and she just tells me it’s because his skin is very sensitive to pee, and it irritates his skin. She said it’s not a yeast infection. Does anyone have experience with a baby who has to wear diaper rash cream 24/7?