Why do some men ask for nudes? Watch
And are you over/under 18?
If you're under 18 it's a crime to send an indecent image of a child - even if it's of yourself - and a crime for him to ask for one.
If you're over 18 then I assume you're at Uni, so if I were you I would seriously ask for my fees back as it's obviously not learning you anything important.
Trying Googling - grooming, sexual exploitation, catfishing, revenge porn, sexting...