In the sixth season opener - Charlie discovers than an ex of his has a son that looks almost exactly like him and that his name is Chuck. lol. Short for Charles.
So, Charlie is trying to find out if Chuck is his own child. His ex girlfriend would not tell him. So, it is killing him inside knowing that the child out there could be his own. Charlie then goes to his ex girlfriend's apartment and give her money for kind of Child Support and tells her that he will pay her every month. He leaves the apartment and a women comes up to her apartment and takes Chuck. His ex girlfriend wasn't the mom of Chuck. LOL