Hi kelsey, Antidepressants are often used in PM and are notorious for causing ED, POpiates can also cause lowered tesstosteron in men and probably women which is what controls labido. ED meds only address blood flow, they have nothing to do with desire. If his testosterone is low which happened to me from taking opiates, no dose of cialis or lavitra will give him drive or have any effect on labido. Fortuntely it;'s as easy as a blood test and if your doc is thourough, he will do a MRI of the brain to check the pituitary for tumors, Most P tumors are benign but hey can damge the the gland or cause it to not work properly andand regulate hormone levels.
This started for me with methadone but has come and gone with the change of meds and the change in delivery kethod. Fortunately there is an easy solution which is to supplement low levels untill he's back in the normal range. This can be done with injections or more stable can be achieved with daily aplication of topical Androgel or a patch. BetweenAndrogel and excercise which is great for labido, I have scripts for that stuff I wouldn't bother filling. ED treatments only work on blood flow problems, labido is a hormonal or emotional problem. Hopefully levels can be supplemented back to normal for his age. I'm about to turn 41 and I'm no where near puting that chapter of my life to rest.
Good luck and give us a happy update if you find an answer.

Take care, Dave