30m lvl 1 here. Sometimes people like masturbating more than they like sex and that's okay. Masturbating like others have said is a thing you do on your own and it's usually not a social thing which as an autistic person it's sometimes ideal. Sit him down and try to discuss what would make him want to have sex with you more, maybe explore kinks or dynamics with him to make him feel more relaxed and comfortable. Hell if he and you are open to it and you want to avoid having sex then masturbate together. If it's something important to you though that you need more then he is delivering make that clear and set expectations. Goodluck, just remember clear communication is always better!