it may be something as simple as allergies, of it could be a scratched cornea...if your dog is a breed which has protruding eyes,such as a pug or bostonterrier, it is likely he has scratched the eye, also if you go on the beach,(i live on the water and my boston had this happen) he may get a piece of sand in the eye that will also scratch the cornea. a cat will do it to. also hanging the head out the car window is not a good idea. he may also have an eye infection. dogs get a lot of the same infections we do ,such as pinkeye. the only real way to determine what has happened is to see your vet. and do not wait too long,i would try for tomorrow the longer it goes,the worse it will become, and some cornea ulcerations do require surgery, if mild it will only require anti-biotic drops 2 times a day for 12 days. lets hope for the best!