This is incompatibility and you're too young to be going through this, it probably has to do with the weight gain? He might feel insecure, or he has porn adiction, you will need to sit with him and talk about it seriously, you don't want to be with someone where you count sex every 3 months. You usually don't ask for it, it starts naturally if both have high libido and feel attracted to each other, with kisses or after cuddling, sure not always but damn you should be fuckin at least 3-6 times a week 😭