So I like this guy his name is justin and he just graduated from 12th grade but I am afraid to tell him because he is my older brothers best friend and he might not feel the same way so should I tell him or let him go

Let go. Brother's best friend is usually very risky unless you both feel the same way.

Furthermore, he's off to university and he's going to start a new life.

Since you are in high school, it's better to find someone your own age.