Need some advice

My boyfriend and I decided to move in together. I moved to a new city for my job and he moved in with me since were both young and it works. When we first moved in it was sex almost every night. Or randomly during the day. It was alot of fun. But for the last 3 weeks it's been maybe once a week sometimes twice. I've tried suprising him and he says hes not in the mood. Is there anything that I might be able to do to fix this? I've tried talking to him about what's going on and he doesn't tell me much just that he hasn't been himself lately. Any advice would be appreciated