What would be the best way? my bf has been in the Army of Belgium for almost 9 years.?

My bf is a corporal in the Army and he has been in the infantry for 5 years. He is a soldier first cl in Belgium. He wants to live in the States with me and join the US Army. We have gone to the INS office and they could not do anything because there is no special way for military personnels to move from one country to other, although Belgium is the head of NATO that does not help my bf at all. Well, I was thinking about him applying green card here, so that he could be a permanent resident and join the US Army but I don't know what steps we need to take to apply green card for military and all other requirements. Anybody can help me?