I caught my little brother masturbating?

My name Is michael Asked: I caught my little brother masturbating?

Hi , Im a 16 year old guy and I have a 14 year old little brother. Anyways , Me and my brother share a room (only for a while , im getting my room re-done and stuff) right now andI have my mattress on the floor right now while we sharing it and he is in his own bed but Im all the way across the room from him. I woke up today and heard moaning and when I looked over he was masturbating and I said uhhmmm.. do you mind??? and He said "shhhh!!!!" and didnt stop and then I left the room and shut the door
I dont understand why he didnt stop ,I mean if he walked in on me I would have stopped
What should I tell him?
Its been really awkward
and I will not tell our parents