How old are you? It changes a lot.
My thoughts from a comfortable armchair, taking time to think about it and not blind sided by the emotion of a family member on the spot: Promoting feminism for the good of society and promoting yourself for the good of yourself are two different, but often aligned, goals. Right now, you're talking about promoting yourself for the good of yourself.
Yeah, there are a lot of people abusive to their SO's. That includes women being abusive to their male SO's. That shit needs to stop, I'm not arguing against that, I don't think you would argue against that. You know yourself, though, and (I'm assuming) you would never do that. So women hitting their boyfriends, yeah, it's a problem, but it's not a problem for you. You have full control over that situation, and the solution is dead simple: Don't hit people. Women abusing men happens, but you will never be in that scenario.
Men assaulting women though, that is a problem you need to be prepared for. The end goal for feminism here, for the problem to be solved, is for every man to respect boundaries. That's not saying all men are rapists, if 999/1,000 respect boundaries, there are still people that are a problem for you. It doesn't take every man in the world to come together for a rape to occur, it just takes 1. I'm sure everyone banding together about despicable that 1 was is real comforting after the fact.
I'm assuming you're older than him. 13 year olds are dumb, but you can't just assume he will grow out of it. I would ask him why he's so passionate about this, and why he is so worried. Don't invalidate his feelings, or try to argue which gender, or which type of assault, is worse, or more common. Just make sure that he understands that yes, sexual assault happens, and that you do not want it to happen to you.
Also, good luck trying to learn solid technique from a 13 year old. It takes a lot of mat time to practice with full resistance sparing to get a technique to work, and it takes a lot of practice to be able to reliably fight outside your weight class. I'm a man, and there are women at my gym who have been training a lot longer that can dominate me, but my size + strength difference does let me overpower women who have a smaller skill advantage.