How to let go/know friends?

Throughout the years there have been many changes relationships wise. It's so hard to choose from who my friends are today so many people are changing. One thing that i have no control over is them moving away, i've lost about 4 good ones from that. But the ones i can control like who are my real friends is extremely difficult. One of my good childhood friends always uses me as a back burner. You know if his plans fail he will call me. Oh and anther night we were all having a great time (childhood friends) at a fair.. but since he drove he told us when to leave. We left at 10 ( we normally stay out till whenever we want)... but the reason was that he wanted to drink with these kids which really angered me and my friends. So we went home at 10 while having a great time bc he wanted to go drinking with some "cool" people. The kid is easily influenced. Anyways is there a method or formula of who to hold on too?. I have in counting only 4 dependable friends. Btw im 16 turnin 17.