what can be causing my dog's eye to be discarging and irratating him?

My dog's eye is red and constantly running and he keeps scratching at it.

Could be an injury - like a cat scratch (my dog had that happen once, needed antibiotic cream), or could be grit or something stuck there. Go to the vet: it's clearly making him as miserable as a sore eye would make you, plus you might stop it getting really infected and needing an expensive operation later.

No idea, but I never mess with eyes - get it looked at by a vet.

Could be conjunctivitis.Never take a chance with eyes. Please take him to a vet where they will at the very least prescribe and antibiotic eye drop, and possibly put a dye in the eye to check for scratches. In the meantime you can use one teaspoon of salt to one pint of boiled cooled water and flush the eye from cotton wool. This will help any infection setting in UNTIL you see the vet.

The vet will be able to tell you. Why haven't you already had this dog to the vet. You don't futz around with eyes. Vision is precious.