Eye Issue

My almost nine year old sheltie, Henry, has been having an eye issue. The third eyelid is showing and his lower lid (conjunctiva?) looks very puffy. It's not cherry eye because it's the lower lid, not the corner of his eye. We took him to the vet who ruled out eye infection, injury, and dental issues. We were given antibiotic drops for him which we have been using for almost a week with no improvement. I'm afraid of the costs of expensive tests and specialists, so I'm wondering if anyone has seen this in their dog, what it was, if it resolved, and what was done to resolve it. I've read about Horner's Syndrome (which sounds/looks like a possibility) and it usually has no known cause and resolves on its in weeks to months. Has anyone experienced this condition in their dog? However, Henry has a history of itchiness and eye and nose discharge, so I suppose this could be allergy related, although benadryl hasn't helped. He does paw at it, so that seems to indicate it itches/is bothering him. We've tried ice packs as well. Thoughts? I would include a picture but I don't have that option.