First of all, hon, you really need to get a handle on your relationship with your girlfriend's son -- especially if you intend to marry her or live with her.... one of you has to be the adult... and i would think that person is you.... so be a good example. The kid is probably a brat because he's obviously gone through his parents' separation, and put yourself in his place -- he probably FEELS ABANDONED!
And here you come along to take his mom away from him...
It's not true, but this is how kids think... so if he is resentful, you can remind him you know how much his mom loves him once in a while...
Meanwhile, if you dont' want kids of your own, do not have any... it's not abnormal for some people to be disinterested in raising a family. and to be quite frank, if i had to do it all over again? I would not have had children. I have two sons, love them with all my heart.
I have a good friend who doesn't want kids....and she's a wonderful person...
take care.