I am sorry your boy has this issue. It is most likely his anal sacs:
Apply hot, damp compresses to the affected area, then wash with 3% peroxide and use plain neosporin all three times a day.
If it does not look better in a day or so he may need them emptied by a vet or groomer and an oral antibiotic.
Please let me know how he is doing.
I hope he is better very soon.
Best wishes for him,