Why are my dogs eyes red?

I have a mini doxie and his eyes are red. He keeps rubbing his face on the carpet to scratch them. He is producing a lot of eye boogers. I was told to give him benadryl because its most likely just allergies. I did but they are still red and hes still trying to itch them. He does have a history of heartworms but has been treated and is on a monthly medicine for it. Also he has been outside often and i live in fl and the weather is changing. Which leads me to believe its allergies but im a worried first time dog owner.