Based on the OP’s own post, his girlfriend IS treating his son as “less than.” That’s the problem. I did not say the girlfriend has to think of the boy as her own or have him call her mom or anything, but if the two children live in the same house, they absolutely should be treated equally. They should both be shown the same kind of outwardly loving gestures and have the same privileges and responsibilities. Even if the girlfriend feels more love for her own child internally, outwardly, she should treat the children the same. To do otherwise is to damage the child. This is especially true since his own mom seems to have problems and is unable to give him the love or attention he clearly needs. When I have dated single parents, I do not pretend the child is mine but I do treat the child with love and it is genuine. I would not treat a partner’s child differently than I would treat my own child— especially if I had been there since the child was a baby. It is different if your partner has adult children who are already raised. My (now ex) partner had a 7 month old baby when we started dating and I treated her as I would my own child— without pretending to actually be her mom.