Make sure that you find a vet that works with exotic pets. Rabbits are considered exotics. I had a rabbit before be misdiagnosed because the vet said that he treats rabbits, but it turned out that he know nothing. I have had rabbits with watery eyes, as Hermelin explained, it turned out to be their teeth. A vet with little rabbit experience once diagnosed one of my rabbits as having an eye infection, and it was his teeth. I had one rabbit with a watery eye. He was always digging in his litter box, and would get dust in his eyes. When I made him a scatter proof litter box, that ended his problem.
My rabbits with teeth problem usually would stop eating their pellets, and that was the first sign of teeth problems. I did on occasion have a rabbit with teeth problems that did not stop eating and I didn't catch it until the eyes watered. That rabbit also showed signs of pain with behavior changes. If it is from teeth problems, he should be seen by a vet asap.
It could also be that he has clogged tear ducts and the vet would need to flush them. To make him more comfortable, you can rinse his eye with eyewash from the drug store. If the problem doesn't clear up after using eyewash, he needs a vet.