
I had something similar when Buster was about 5 months old.
He had just woken from a morning snooze and his eyes looked red & sore, one lower eyelid was drooping so much so that you could hardly see he had an eyeball in there. He was aslo acting dopey (more so than normal LOL) and appeared to stuggling to walk as if he was drunk.
I immediately phoned the vet to tell them I'd be bringing him in as an emergency. While I was waiting for a lift to the vets he also started to salivate (way beyond his normal slobber).
I thought he was going to die!
At first the vets thought it was poisoning

His eyes were checked then & there for signs of problems as his eyes seemed to be too painfull for him to open., nothing could be found but due to his very aggitated state, they kept him in to run some tests and examine his eyes more.
I got a call form them a couple of hours later to say he was back to normal apart form a slightly sore looking eye and that they could not find any cause and could only assume he had a bad reaction from an insect bite/sting.
It took a couple of days for the 'soreness' look of his eyes to go.
I tore my livingroom apart (where Buster sleeps) looking for any possible insect-like culprit but found nothing.
About a week later it happened agian, but without all the 'dunken' type symptoms, it was just his eyes effected, so I didn't take him back to the vet just monitered him. Sure enough, within a couple of hours his eyes looked better and, fingers crossed' it's never happened agian :)
(he's 3 & half years old now)
You've done the right thing going to the vets.
I hope your boy is fine and please let us know how he is.