After taking various opiates/opioids for years, I had my testosterone checked, as I too had no sex drive..and my blood level was 50 ng/dl, whereas the normal male levels are 300-1000. From what I've been told by my doctors, this is not unusual, and now have a replacement script along with all the others.
So yeah, opi's will ruin sex drive for sure, but it's treatable. Keep in mind that if he decides to go that route, and continues to take a lot of oxy, his sex drive will probably not return to 'normal,' but it's worth a try.