Aggressive bf?

I am a male 16 (almost 17) I was hanging out with one of my girl friends. We basically went to the city to buy some stuff because one of our classes ended short. Well moving on to the point. I got 2 friend requests on snapchat later that day. I got a message asking if we hung out earlier that day. (This was pretty late). I mean I assumed he already knew so I just wrote yes. And I asked the girl who this guy was talking about and who this guy was. She basically said don't talk to him etc... and that I guess it's her bf's brother. Well he kept sending me messages telling me like ''you know she has a boyfriend right?'' I just told him I have a lot of girl friends and that it's nothing to worry about. Well he ended up telling me to stay away from her and another friend of mine. I asked about it and she told me to pretend like I understood. We talked back and forth since I feel like that and she told me it was really up to me how I would like to handle it.

(update) her bf just told me to meet him at lunch time. I am sick as well so if he wants me to fight this is really bad timing, legit not an excuse I am making up lol. I have been sick for the past 4 days.

Now for my question. I have never experienced anything like this. How should I handle this?