My present and second marriage was a "mixed family" situation. The ones to cause the greatest problems in the adult relationship are the kids; if not all, there will always be one.

Your boyfriend's 18 yr old son doesn't seem to accept reality for some reason. Maybe Dad hasn't given him enough to go on so that he CAN believe it? Your boyfriend isn't doing his son or other kids any favors by "staying with his ex"... if indeed she IS his ex.

If indeed your bf loves you as much as he says he does, it's time to tell him to "don't tell me... SHOW me!" by not staying with his children's mom anymore. I agree with the "Watch the feet" comment. He needs to have walked away from the ex and towards you quite some time ago.