Help! I don't wanna lose my best guy friend?

So my best guy friend (been great friends since we were little) ( he's 16 & im 14) has recently told me how he feels about me. Only problem is, I'm in a relationship (6 months) (hes 17)and he knows that & had met him a few times. he wants me to break up with him and choose him instead. I love them both dearly, but in different ways. I love my best guy friend in a big brother/family way & I love my boyfriend in a much stronger relationship way. How do I tell him that I just wanna stay friends? and that I don't want our friendship to end because he is like a big brother to me and I know I can tell him anything or go to him for advise. How to tell him with out losing him completely out if my life? Please help :(