Father whos cheated on my mum and has a son from another relationship?

It's possible your father has a lot of regrets in his life...and everything you say and do only reinforces theses regrets. This is not your fault , your dad made choices and now has to deal with the consequences ...continue to be his son but don't allow his negative attitude effect you. ..rise above it and be the better man. Remember you are his son and he does love you even if he is not very good at showing it.