Sounds like he has "stud tail." Here is from another website:
Stud tail in cats is usually easy to spot. Common symptoms include:
Fur becoming greasy around the tail and back
Blackheads visible around the tail
Pus present around the tail
A discoloration of the tail fur
Losing hair around the tail and back
The cause of stud tail in cats is the animal’s sebaceous glands producing too much of the oily substance sebum, which clogs the pores. This is a hormonal effect that is especially likely to happen when a male cat who hasn’t been neutered reaches puberty.
Your veterinarian will conduct a thorough examination of the affected area. While the condition can be easy enough to spot, it’s important to rule out other skin conditions and ailments.
When it comes to treatment, it’s key to clean the affected area and then keep the sebum production levels lower. Your vet might prescribe a special antibacterial shampoo to help keep the tail area clean.
In some cases, the vet might also prescribe some sort of topical medication too, similar to the way humans use acne creams. But never use topical human medicines on cats or other animals.
Once your vet has cleaned the affected area, he or she might also recommend neutering your cat.