It's not anything illegal which is good, it's a bit strange and maybe worth a conversation if you feel so strongly about it.

BUT, it's just an anime cartoon, there are a lot of people who like the pornographic version of anime, not my thing but to each their own. It's a big jump to assume from this that he is attracted to actual underage people. An accusation like that could ruin a life. So don't just throw it out there on a hunch

Millions of people watch hentai, the porn version of anime. The vast majority are not actually interested in school children, millions of people watch porn where women role play as school girls, again, it's not illegal and the majority wouldn't actually engage with someone underage. We don't even know for sure if he was thinking of the show when he was masturbating, it could have just been on in the background

If you bring it up he'll probably get very defensive, this doesn't prove anything though. If you were accused of being attracted to children when you aren't you probably would not respond very calmly, it's not something anyone wants to be linked with, and if you were, then you respond in a similar way but it would be lies to try get away with it.

So, in your position I'd try to look in his search history, or his bookmarks, look at who he follows on media platforms maybe. If everything seems OK then I'd leave it there, if not, obviously report to the necessary authorities, don't tell him anything as he will have time to get rid of evidence if you do