"No, he is not my boyfriend."
Translation:Non, ce n'est pas mon petit ami.
Because it is considered as an exception of exception and it lacks context to fully justify it. So, let's stick to the rule: he is not my boyfriend = ce n'est pas mon petit-ami.
So, please go back and read this: http://www.frenchtoday.com/blog/cest-versus-il-elle-est
I think "ce" translates rather "it's my boyfriend", using he implies it should be "il".
In Quebec, we have another word for boyfriend: chum. 'Non, ce n'est pas mon chum'
Please read this: http://www.frenchtoday.com/blog/cest-versus-il-elle-est