I would need more information such as age, neutered/not neutered, and examples in detail of his behavior.
Are you sure he doesn't just want to play? Usually a dog will give some sort of warning to another before a fight. This sounds like play to me.
In the meantime you can do a search here at Boxerworld using search words like "aggression" and "aggressive" and you will find TONS of useful information on this subject. If you can't find your answers there, post again.