Realistically, a lack lustre sex life is usually due to something being wrong in the relationship. I would suggest you have a chat and find out what is going on. Is he depressed at work? How is he feeling about other things? How does he feel about life? How does he feel about the future? And how does he feel about the relationship?
You need answers and you can't offer support if you don't know what's going on. Try not to take it personally, this is what's going on for him.
All the best,