First thing to remember is that some guys just have a low sex drive - once or twice a month is fine for them, while other guys need some sort of sexual release three or four (or more) times a week. I guess I would caution trying to increase libido if that’s his “natural” way.
If that’s not the case and you have noticed a definite drop in his sex drive, then maybe you start by looking for underlying causes. Is he depressed? worried over something? not feeling good about himself for some reason? Is he taking any prescription drugs (or otherwise) that could have an effect? There can be many causes and some are not readily apparent. It’s good to start there if you have noticed a change.
There are some therapists who suggest that half of a man’s libido is his partner. A partner who loses interest in themselves may not realize the effect they have on a guy - so that’s another area to take a look at and see what might be happening in that regard.
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