<human>: Can you provide a brief overview of the text?

My boyfriend and I have been together for about 2 years, and we have generally had a good sexual relationship. This winter, my boyfriend basically seemed to lose interest in sex. We went from having sexual contact every couple of days to having sexual contact every couple of weeks. Right before I went on a two week trip this April, he admitted to me that he was bored with our sex when we did have it. We didn't talk about sex at all when I went on the trip, but when I got back, his libido was right back to normal.
 We started having sex frequently again, and this lasted for about a month. Then, his libido went right back down again, and our sexual activity got very infrequent. I tried to spice things up and do all kinds of sexy things for him, but I hardly ever got a reaction.
 When I try to discuss it with him, he gets frustrated and won't discuss it. He's assured me that it's not my fault. He says he doesn't even want to masturbate anymore. He says he doesn't know what the problem is. We've been apart (because I have an internship this summer) for 3 weeks now, and every time I bring up anything sexual he changes the subject immediately. I don't know what's going on. Other aspects of our relationship seem great. This is such a drastic change in him. It used to seem like sex was always on his mind. 
Does anyone have any ideas as to what the problem might be. Is there anything else I should be doing here?
<bot>: Used to have great sex life with boyfriend, now he has lost interest in sex and doesn't want to talk about it.