Partners lack of sex drive?

My boyfriend and I have been together for 3.5 years and I've recently found that he's less interested in having sex. I've asked him about it and he didn't have too much of an explanation, just that he didn't feel like it as much?
We only have sex 1/2 times a month which bothers me. I am understanding and don't pressure him if he's not it the mood but it's hard when we are on two different levels. 
We used to have sex usually 2-3 times a week up until 4-5 months ago. 
Has this drastic change ever happened to anyone else?