Stress is the first thing that came to mind. If you are putting pressure on him to have sex, that alone could cause stress about the very thing. Has his work become more stressful? Is he working more hours? Exhaustion can be another thing that could cause a drop in sexual desire. It could also be hormonal. Ask him to got to the doctor and have his testosterone levels checked. Another thing could be some medication. Some medications, even one comes to mind for heartburn can cause a drop in sexual desire.
You need to talk to him about this, not in the bedroom, or at bedtime, but a "safe" time when there is no chance for sex to happen, so he won't feel the pressure. When you talk to him, you need to ensure you are not sounding like you are complaining to him, but truly interested in what has changed to cause this drop in desire. If you approach it in the right way, he may very well open up and let you know, if he knows at all. Again, have him go to the doctor and get checked to ensure it is nothing medical, and have him ask the doctor about any medications he is taking, and then, go from there. Sometimes it could be something as simple as the two of you getting away to a motel, even in the town where you live, to liven things up.
Communications is the most important thing, and with communications, comes listening, just as much as talking. Slow down, talk in a non threatening way, and do it at a time and place where he knows you are not just attempting to coerce him into sex at that moment.
I hope this helps. Good luck, and great sex!
