oh dear, what messes we get ourselves in, and we don't even mean to.... Anyway, I feel your pain..I am in the same situation.. I've been in the relationship for about 9 years (engaged now) and it is similar. What makes a guy stop wanting to have sex????? Especially if one is still attractive and intelligent! I think more and more men are having a difficult time adjusting to independent women..
Do you know for sure that nothing else is going on? Maybe he has low testosterone. You could just go buy some herbal concoction and "drop" it in his dinner. Good Luck, and remember, if wwe only live once, make this one count!