He's different when he's gone

So me and my boyfriend have been together for two and a half years. We met in college and fell pretty hard for each other, and that,s why we're still together now. The catch: he's an international student. So in the summer, he goes back to his home country for 3-4 months. When he's here (in the same country as me) he's so wonderful. He treats me like i'm the most precious thing in the world to him. I couldn't ask for a better partner. But when he goes home, he changes drastically. He becomes this person i don't know. For example, he sometimes becomes irritable just from talking to me, when I've said nothing wrong or hurtful. He's always busy so we don't talk as much, he doesn't seem very interested in me and he's very vague when I ask him what he does when he goes out with his friends... It upsets me a lot. I just don't know what to make of it.

He says that he's really busy with his full-time summer job and that stresses him out as well as other more personal problems (which i try to help him with). I haven't gone with him mostly because i also have work and I'm already paying for two vacations this year and don't have the money to visit him. But he's never really asked me to go over, i just assume it will happen in the future. He talks about us moving in together when we graduate and settling down in one place, but it's starting to look more like a fantasy more than reality due to other life-committments that are likely to pop up in the future.