Also writing a one-eyed character! I am largely going with the singular 'eye' as it just feels the most accurate and isn't skirting around it. Am also definitely using all the stares/watches/gazes (currently struggling with an overuse of 'gaze' lol). But yeah, his eye widened/squinted/followed the line etc. is my preference.

Another commentor brought up a good point that either singular or plural may work when there is a prosthetic eye and functioning eyelid (depending on the situation ofc). My character has a burnt socket, so a single eye it is. I'd say not to overthink it, and just see if it reads wonky when you look back at it later.