I turn 16 in October and my grandma told him I was actually 15 but I played it off like she was confused and got it wrong. I'm so scared...

So you started a relationship with a pedophile off with a lie.?

A real relationship is based on truth and communication. I would hope you have good parents or guaridans hwo will make sure he stays away and protects you

You have no business having a relationship with a 21 year old. Even if he thought you were 16 he should not be involved with you. You need to break it off.