Anybody have this problem?

My boyfriend and I have been together for a year and a half and everything's been great. But for the past month and a half, he has not wanted to have sex. Like at all. I try and start it, kiss him on the neck, touch him, etc. But he just laughs and pushes me away. He makes a bunch of excuses, says we don't have time or something stupid. It's really frustrating. The last time something like this happened, I talked to him about it and he just said sorry and tried to have sex right that second. I felt like he thought he just had to. I don't know what to do or think. Any advice would be appreciated!