Hi there, this is Dr. Christie! I am a licensed veterinarian and would like to try and help you with Max.
Three possibilities I would consider for this abscess: First would be that of an abscess, secondary to trauma in the mouth (bit the side of his mouth, scratches/cuts from grass/hay, etc) that allowed bacteria and debris to migrate into the mucosa and eventually form an abscess. The type of bacteria could potentially cause it to be the dark smelly liquid produced.
Second possibly a hematoma (blood and bruising produced from some type of trauma) that may have gotten infected.
Thirdly, cheesy glands is a possibility given the location but usually the material produced with this is thick and like cottage cheese. While this is lower on my list, I would still be cautious until we know what the exact cause is! I would keep the goat quarantined, use good hygiene, etc as the bacteria is readily present in the pus and is highly infectious and can last in the environment for up to 4 months.
I would recommend getting your vet out to take a culture to rule out caseous lymphadenitis (cheesy glands) to be on the safe side as it can spread to the rest of the herd!
Here is an article concerning CL if you need more information concerning it! http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0008/178352/goat-cheesy-gland.pdf
Please let me know if you have any further questions or concerns!