I wouldn't say Hentai characters look young - more like petite. It would never have occurred to me that the cartoons could possibly be depicting underage characters. I suppose there are people who would argue there are more obvious issues with Hentai - like beastiality, incest, rape, etc etc. but it can't all be tarred with the same brush in the same way that regular porn can't. Have you actually seen the particular videos he's watching? Maybe that would put your mind at ease but if you already know there's nothing exactly wrong with what he's doing then I guess you just have to accept that it's none of your business and deal with any residual feelings you have independently. All the best.