Hopefully I can make a suggestion. Is it perhaps that he may be simply bored with what the 2 of you normally do. Maybe try something a bit different, some role playing, maybe a little light bondage,

and sometimes it can be things you do out of the bedroom as well. Maybe cook him a meal that you do not normally eat at home. Offer a massage with scented oils... and so on. Sometimes guys are more receptive if they feel the partner is doing things for his pleasure as well as hers and vice versa..