Hi, Aimee,
You dachshund has experienced an Anal Gland Abscess, which has burst, as abscesses usually do, to the nearest point on the skin.
Presumably his other Anal Gland was removed because of repeated blockage, causing irritation. The remaining one has blocked again, and an infection has set in, causing the abscess. Let you veterinarian see him. He will be able to clean out the abscess cavity and maybe give him some antibiotics to control the infection. He may be able at a later date to remove the anal sac so as to prevent a recurrance, but that is more difficult after it has abscessed because of local adhesions occurring.
Walter