Meeting his mom 😳😖

This is long so please bear with me, I need help.

I'm meeting my boyfriend's mama tomorrow at his graduation party. She doesn't know we're together, and we've been dating for seven months. He hasn't told his mom because we have an age gap (I'm fifteen & he's eighteen) and he knows she wouldn't like us being that far apart (she's actually "banned" him from dating me, but that was after we were together for a couple months). In my state, our relationship is 100% LEGAL.

I discussed it with my babe, and I told him that if she asked me about us or our relationship, I would NOT lie to her. I would tell her anything if she asked with exception to us having sex (because we are, but no one tells their parents about that) and that he's snuck me over to his house before (because I know how she feels about that). Lying to her now would definitely cause problems down the line, and I'm NOT putting any of us through that.

I'm intensely stressed and scared about meeting her and of what to say to her. I'm going to be introduced as just a friend, so I don't want to come off as too overbearing or "overly interested". However, I do definitely want a chance to talk to her. He tells me she's VERY stubborn (just like he and I both are) and set in her ways over the age gap thing, but I feel like if I have a good conversation with her, I can show her I'm not a little shit fifteen year old that's playing games. I'm seriously here for her son, and I would do anything to prove myself to her. I know where I want my life to go; I'm 6th in my class of 379 students, a straight-A student, and I play two instruments. I'm very mature for my age, and I have my shit together as much as a fifteen-year-old possibly could.

Does anyone have ANY advice on what I can say to her or anything I can do tomorrow to make her rethink her opinions? Anything I should or shouldn't do? If you were in her shoes, how would you feel about your son's girlfriend telling you they're in a relationship versus your son? Should I be completely honest with her and tell her WITHOUT her asking? If she did ask me, how should I tell her without making her freak out?

Thank you all in advance, I know I need the help and the support. It means so much to me.