Mental illness, sexual

there’s one person in my office which we had considered as weirdo or may i said silence pervert right now. firstly what happen is he often not understand what we ask him to do which we had to explain everything to him repeated. Frequently. after sometimes he become worse as he can understand simple task. can’t even remember properly. firstly we just think that it his personality to be quiet and make his own work. but last week we found out that he make pornographic comic. and after some investigation by us, he actually express the character in the story(which very close to our character). but he dont know that we know. what i know is he often open live chatting about sex and make it to improve his art. he had collection of sketch for character and the position in sex for his comic. and somemore has had specific character for only Muslim girl and women. thus this consider as mental illness. what kind of mental illness was is? is this consider dangerous.