I love this boy but he is going out with someone else I have had a big crush on him since grade 2 What do I do?

Well you have 3 options:

a) If you are good friends and you think he might have some feelings for you confess that youu like him and more than likely he'll say he likes you back.

b) This is a bad option which I don't recommend, use only if you're desperate. You could try to sabotage their relationship by making him cheat on his girlfriend and spreading rumours, but again, I advise against this.

c) Wait and see how the relationship goes and tell him afterwards how you feel. If it is serious, then I wouldn't want to regret never telling him.