Air superiority is a beautiful thing. Stealth bombers have a range of 20. It's ridiculous. The great war bomber only has a range of 6, but that's still a huge help since you'll be playing defense. Since you're ahead in tech and he's being more aggressive I'm assuming he doesn't have Radio yet? Without Anti-Aircraft Guns you can probably just bomb his advancing army into oblivion before he's close enough to do any real damage to you.