I think there is a wide range of sex drive in men. Your guy sounds within the normal range. Some guys just have a stronger libido than others. I've experienced both ends of the spectrum. What the low libido guys don't like is to feel they "should" be having sex more frequently (makes them feel deficient), so it can be touchy talking about this. But the fact that your guy started out with higher libido and then went to low does sound like there is something else going on, as you have said. Hope everything works out for you two.