My girlfriends oldest son?

Okay me and my girlfriend of 4 years have a 2 year old son together and she has 2 kids from a previous relationship 5 and 7.Here lately we have been fighting over her oldest son about his attitude.His mom always gives him his way and cries every time for no reason just to get it.Well i finally got sick of it and told him to stop being a crybaby and he starts crying about it and of course my girlfriend defended him and said to me to quit calling him names.I just feel i have no say what so ever and it's really getting me upset because i do everything for all 3 boys and that's how she treats me.She just tells me ooh that's just how he is.I just wish he act like 7 year old instead of a 2 year old.Another thing he does that really is annoying is he will say sorry a billion times for something he doesn't do and then pout??Am i wrong on this subject and what do you think i should do?