That's nice MargueritaPink. 
Op 13 is a good age to start to talk about these things.
If they're asking what a feminist is, films like the one I linked will help to point out that women and girls are often treated unfairly by society. I'd so of say "ok so you were wondering what a feminist is - watch this: .... what is the film about?" And so on, just letting them understand inequality themselves and just calmly talking about how in the past women couldn't vote etc. They don't need it all at once, just a good little opener or two.
Don't comment on his opinions but I'd mildly point out that everyone has different opinions on all sorts of things. Eg which football team is the best, if Nando's is any good etc. That's his opinion. Your opinion is that feminism has done a lot in the world to make things more equal but still has more to do, especially in many parts of the world where women are still given fewer rights than men. And also that there are men and women working to do this together.
I'd probably also try to squeeze in other political things eg racism and key historical moments there as a way to paint a picture of how there are different movements that evolved to tackle inequality in different areas of society.
A quick google indicated quite a few films that's are supposed to be 'feminist' so I guess watching those together (not overtly, just by the by and then chat about key themes afterwards)
I'd do little things like this gradually and innocuously. It's just occurred to me that feminism isn't an area outrightly discussed in the curriculum as far as I know. Perhaps it should be.