He often talks to you about what happened in the bedroom
For an ordinary friend who has no interest in you, they don't talk to you about what happened in the bedroom, because it will make you both feel embarrassed. But if a friend of yours is constantly talking to you about what's going on in the bedroom, it's probably because he wants to test your attitude towards casual sex. If they really want to develop a serious dating relationship with you, they will definitely not talk about such things with you. They do it for two reasons. The first is that they want to communicate indirectly that they are just interested in casual sex and are not going to join a committed relationship. The second is to find out if you are interested in such a relationship. If so, they may consider inviting you to a NSA relationship. If not, they can avoid the embarrassment of being rejected by you.
He just contacted you at night
This is a very direct signal that this person wants to establish a FWB relationship with you. If this person only contacts you at night, it means that he doesn't want to enter into a committed relationship with you at all, but is only interested in your body. Those who really like you will greet you in the morning or contact you during the day. And those who just want to be friends with you at the same time, who have sex with you will choose to contact you at night.
He said he was not interested in dating
The most direct sign is that he will tell you frankly that he is not interested in dating. But at the same time, he keeps contacting and approaching you every day. That means he wants you to be his FWB. If you can't accept this kind of casual relationship, you can gently tell him that you are not interested in this type of relationship.