I need some advice!!!

My boyfriend and I have just celebrated our 6 month anniversary.. Not long I know, but something has changed and I don't know if I should be concerned. We've usually been having sex quite frequently, at least 4 times a week. But lately it's been like never. Last month we've only had sex twice and none this month! He seems completely un-phased by this, like he doesn't even notice. What should I do? How should I handle this?