I got the histopathology ("the microscopic examination of biological tissues to observe the appearance of diseased cells and tissues in very fine detail") results of Mason's removed eye today. The vet said it looked like he had primary glaucoma, and his ophthalmologist confirmed it. His ophthalmologist suspected he had secondary glaucoma caused by the mature cataract in that eye, so she was surprised to hear it was primary. She told me to use the left over Dorzolamide eye drops from his other eye on his one remaining eye twice a day to help delay the inevitable (my wording, not hers).
The ophthalmologist mentioned Endolaser surgery being an option in his remaining eye (...for $8,000). It has an 85% chance of saving whatever vision he has left and a 95% chance of keeping his eye pressure low. She doesn't perform it until after a spike in pressure because the other 15% is the chance that she accidentally blinds him, which is less horrible when you already know he's definitely going to go blind from glaucoma. She also said that when his pressure spikes it is very important for him to see an ophthalmologist in 1-2 hours. They could then add drops to his routine or use a needle to remove some of the fluid in the eye to relieve the pressure and hopefully save his vision.