Anti-glaucoma medications do have some side effects. Most common and the seeming to cause problem to your dad is dryness. You should start lubricanting eye drops (Ontears gel drops) (CMC 1%) two to three times a day along with other medication.
Next Steps
Do get the intra ocular pressure checked once again as raised pressure can also cause redness.
Health Tips
If problem still persists one can use higher lubricants like Hyla eye drops (hyluronate) or Lacryl Ultra eye drops (PEG and PPG)