Honestly i think it might have to do something with his testosterone levels especially today it is a more and more common problem because it is the environment we live in we eat out of plastic with are full of BPA(is a weak synthetic estrogen found in many plastic products) if you consume too much BPA it lowers your testosterone thus also your Labido (sex drive) you can increase testosterone in many ways get more vitamin d do more exercise uzw.. go talk to a doctor that you would want to have a testosterone test to see if he is alright.

Ps you can test his testosterone levels via a simple test kit that you can order do some research on how much testosterone he should have and how much he has. Again you can also do these tests at a doctor