In general, a man can start losing his libido around the age of forty. It should never become non-existant. But, in some men, it can. I lost mine when I was in my mid twenties, from a variety of issues, mainly from nerve damage.
The best thing you can do is, take him to the hospital for some blood tests. And, if you can, have him see an Urologist? Doing these two things, will save you some anxiety.
I'm hoping for the best possible outcome, for the both of you!!!
Dennis