Need advice..

My partner is only 23, we have 2 children and we do get a lot of time we could have sex but he just doesn’t initiate it or if I do he says no, at the beginning of our relationship 5 years ago we were doing it 3 times a day and now he just don’t seem like he wants to anymore. He doesn’t have a stressful job or anything for him to just stop wanting it so I was just wondering if anyone could give me some advice as to why he wouldn’t want it anymore? He’s young so I would have thought he would still want to. And I know he isn’t cheating on me either.