How Can I Make My Boyfriend Want Sex More?

Dear Dr. Betty,
When my boyfriend and I first met we were always having sex and trying new things in the bedroom. Now it seems like he can go weeks without it and I still want sex every few days or so. Then I get frustrated because he doesn't want to have sex and we have an argument and he gets more stressed out about it. I don't want to ruin my relationship with him. Do you have any advice on what to do to make him want sex more? Or what I can do?
Dear E,