Weight, age, diabetes, depression, high blood pressure, low dopamine, meds, stress, anxiety, low testosterone levels, self-esteem issues... all can cause low sex drive in men. He really should have a physical, maybe even couples counseling can help as well.
Talk to him about being concerned for his health, not concerned for his libido.
This article can help shed some light for you:
http://www.webmd.com/sex-relationships/features/when-a-ma...
Also, it's scientifically proven that men's sex drives are the highest in the 20's and start to wane in their later 30's, while women's sex drives are lower in their 20's and becomes more active in their 30's... interesting.