My bestfriend 16M is closer to my ex 17F than me 16M

Me and my bestfriend were veryyy close and were really good friends. Me and my ex also ended on good times and we were also good friends, slowly my bestfriend started getting distant from me but i think thats bc i kept asking him what him and my ex would talk about and he would talk me but i think he would get upset bc he thought i didnt trust him. In the middle, me and my ex were about to get back together but it didnt happen bc i did something to make her feel bad but it felt right for me. after that, she was upset and called him up after she spoke to her bestfriend, idk why but she did and he kinda was taking her side and was telling her that she deserves better which i can understand bc he cares about her too even though he mightve been my bestfriend. but sometimes i get very upset bc he tells her stuff which he doesnt tell me and before he used to tell me everything. now, after a lot of time has passed, i still asked him once last week if we are still bestfriends and he said we are but it doesnt feel like it at all, and i told him that but he didnt really care because he is the type of person to lose interest when something serious comes up, nowadays he still calls me his bestfriend but it feels like she is and im not bc he even sits beside her now in class and i asked him about that and he said its bc my other friends come and sit beside me before which is mostly true. Now, me and my ex are talking again and might get back together (we broke up only bc of family issues) and nobody knows about it, so now i told her about all this and she said that she thinks he only needs space bc hes the type of person to not engage in serious conversations and the person to leave when u dont respect their boundaries. I dont know what to do, im very concerned and i only want my friend back.