Military spouses help

My bf is in the Air Force he's the most amanizing man in the world. I love him to pieces. He lives on base about 40 mins away from me because I go to college in a different town. Lately he's been super depressed and says he feels numb emotionally. This normally last about a day then he's alright again. He says he forgets all his problems when he's with me. But we can't see each other all the time because of his work and my school/work. How can I cheer him up when he's so down. He says he dosnt know what will help. I try to encourage him and talk to him and idk if that works. I just hate seeing him so upset.