Definitely get his T levels checked. If his levels are low, meds for ED like Viagra really won't help unless he's actually interested in sex. T levels control his labido, Meds like Viagra just increase circulation. The topical is called androgel, I'vehad to use it off and on depending on which opiate I was on for several years. Having low T levels is like someone just flipping a switch and all interest just disapears despite what he may have been like a year ago or in his prime if he's not still there. We have a much longer prime then women.
Anyway, the supplements do work,not just for labido, but men need T to feel like men, to have energy, to build muscle, It's what makes men, men. Daily aplication of the gel is the most efective way to maintain a constant level. There are risks asociated with testosterone, like prostate cancer, shrinking testicles, development of breast tissue and cardio vascular problems, it also has side effects like skin breaking out. There really is no such thing as a benign drug. It's just a matter of does he need it and does the benefit outweigh the risk. Despite the lack of drive or percieved interest, all the understanding in the world doesn't make it less immasculating. It is a major issue and you need to treat it just like you would any other medical problem that's needs attn. There is hope if he can talk about it with a doc. They may want to do a MRI of his brain to rule out a pituitary tumor which are normally benign, but low T levels can be treated.
Good luck , Dave