HELP i am about to lose ALL OF MY FRIENDS. PLEASE HELP ME?

this dude who was one of really good friends is starting to hookup with this girl who is one of my best friends. he keeps blowing me off and lying about plans he has and not responding to texts to me and other people, and only talks to her now. I used to hang out with both of them, and then we all started hanging out together: Me, him, Her, and her best friend who im also really good friends with and have a huge crush on. He only hangs out with me now if i tell him im with those girls, other wise he blows everyone off for smokin weed. I used to think he was real, but i tihnk hes fake and trying to turn my girls away from me and keep them for himself. i dont know how to handle a situation like this and I need help really badly. Thank you!