I'd kinda go with this bit. I don't really admit to my mom that I watch porn. Also different people have different attitudes towards porn, and some are just weird. My sis claims to abhor porn, but she very openly reads a lot of shonen-ai manga (manga with gay love stories) that are a bit explicit.Most guys are not comfortable discussing their porn watching habits, especially not with their mothers. You are his mother!
Also I'd say that the fact that he enjoys anime and that he to his own admission doesn't enjoy porn are not necessarily connected. Different strokes. I love anime and watch several of the series you mentioned, yet I have a...healthy porn consumption.