He was born in the Season 11 finale and the final episode of Shameless.
Frank was initially unaware that Monica was pregnant as he believed she had had an abortion some months previously. Monica who was pregnant with twins had the abortion but didn't do it on the NHS instead going to 'Cherokee Linda'. The termination was only partly successful with only one of the babies surviving - Ben. His twin William was successfully aborted.
Frank only discovered the truth when Monica suddenly went into labour in the Jockey and a horrified Frank did what he always did when faced with a shock... Sit down, have a fag and let someone else deal with it.