One of my dogs won't stop the other. They're both boys. Help?

One is a poodle mixed with no one knows what and the other one is a full breed Irish Setter that my uncle bought online and we picked him up yesterday at the airport. The small one is really scared of the irish setter. The Irish setter runs at him, I think playing, and the small one runs for his life but the Irish Setter opens his mouth when he gets close, although I don't think he wants to eat him. Cause they're not ALWAYS running. But the irish setter also would not stop . He won't stop the small one. And he humps the air and people and pretty much everything. He's 3 months old and the poodle mix is 4 months old. Is there anyway I can get Da Vinci [Irish setter] to stop Mozart [poodle mix]?