I'm thinking of keeping a gift solely for my son rather than splitting it between him and my girlfriend's kids
Why do you think that you just want your son to have it?
My son is more important than her kids ever will be.
Your son is special to you, but all children need to be respected equally and not getting everyone gifts betrays that responsibility of a parent. Go out and buy gifts for all the kids.
I don't give a fuck what you think. Maybe next time my son gets a gift I'll get him a car.
I think that doing this might even just upset your son. You should try to treat all children in your life equally. No one should get special treatment.