Boyfriend sex problem question?

I dont know, desire does often fade over time. He's definitely not worried about anything, especially that he could be keeping secret? No pregnancy scares? It's possible he needs his hormone levels checked as various problems can cause a drop in testosterone, and they are treatable. Though if he is not willing to discuss it I dont know how you'd get him to go to a doctor. Tell him you are worried about his health for some other reason? Worried he might be depressed? Other than that, if he wont talk about it, I dont know what you can do.