Hi! I am sooo sorry for your boy (and for you)! I would suggest that for the time being, you don't let him go outside - even if he begs, at least until you see the vet. It may be that he is allergic to one thing or many things, or maybe nothing... Are you sure he doesn't have an eye infection (what I call 'eye goop')? If he has 'eye goop', his eye (or eyes) will water ALOT and then the discharge will become almost white looking. If it's eye goop the vet can help with that, they will likely prescribe some drops for him. I wouldn't cross the allergy shots vs. pills until you find out what it is and if you really can't avoid shots or pills.
If it turns out that he is allergic to something outside, and you can't avoid letting him out, you may want to consider a 'cattio' or a window addition {screen box attached to the outside]? Make sure to ask the vet, too when you see them about the best way to help him. Good luck with him!
#PetHealthandNutrition