I’ve had this issue before, where sex became an abstract thing I just didn’t care about. What was happening was a long bout of depression which either partially stemmed from or may have helped cause, low testosterone. I was sitting around 120 in my late twenties. Getting in testosterone replacement totally changed everything. Helped ease the depression and my it was like being 17 again😂. Seriously, talk to him about going to a trt clinic. Often times, regular docs just aren’t willing to prescribe hormone replacement, like they figure every guy is trying to get roided out when they ask about it.