dog is being aggressive towards other dogs, is there a way to fix that?

I have been fostering a rescue for the past two weeks and have unfortunately grown very attached to him. I say unfortunately because he gets really aggressive towards other dogs and the community I'm in has a lot of other dogs. I cannot keep him if I he goes berserk every time he sees another one. He's 4, is a great pyrenees, and has not been neutered.