One possibility is a hormone drop, specifically testosterone. Don't feel bad, it drops with work stress, bad diet, age, etc. There are some natural things you can do to boost it. Nutrition wise, he can take some cod liver or krill oil, eat grassfed butter (the Irish variety is best), vitamin d3, elk velvet antler (only from new Zealand or Canada, all others are garbage), and he can eat more eggs and Brazil nuts. For overall nutrition, I would encourage a clean diet. I tend toward the paleo gig but do your own research and decide what is best. Finally, I would orchestrate a basic workout routine. To boost his test, he needs to hit the weights. Specifically deadlift, squat, bench press, squat, and squat. Did I mention squats? Those 3 have the biggest hormonal cascade of all because of the overall impact on the body. The workouts don't have to be long, just work each of those once per week. He can do them all on one day or divide them up. I wouldn't just tell him to go to the gym though, frame it as a fun way to be healthier and sexier together. Do these things and I'm pretty sure his sex drive will go up. Hope this helps