N.,
You probably need to take him to the pediatrician and get some prescription ointment.
I would NOT put any more Desitin on his bumm...don't use diaper wipes, use a warm wash cloth with water only - wipes have alcohol in them that can exacerbate the problem.
He could have a food allergy. My son couldn't have apples - of any kind - no juice, etc. until he was 4 years old. His bottom would break out HORRIBLY - if he drank a fruit juice and it had apples in it? Oh MY WORD!!! torture for him...
My daughter got diaper rash that turned into a yeast infection. She had blisters on her bottom..it was horrible. I felt HORRIBLE...we used baking soda in her diaper and a prescription ointment. We also changed her juice intake from 1/2 to 3/4 diluted and took out acidic stuff....oranges, etc.
I'm sorry your little guy is going through this!! I feel your pain!!