My wife has a son from a former relationship. Dude is a complete barnacle - unemployed, lives in his mother's ghetto apartment, is a sometime drug addict, chain-smoking, abusive, neglectful, 400 lb waste of space. My stepson sees him once a week. Even though his dad basically blows second-hand smoke in his face, has him sleep in a sleeping bag on the floor at his house, sometimes doesn't have enough food in the house to feed him, and will often leave him in the care of his grandmother (on his one night per week) so he can go drink/smoke, my stepson runs over and gives him a gigantic hug and tells him how much he loves him whenever they get together.

I stay healthy, have a caring relationship with my wife, have a great career, do not smoke, drink, or do drugs, own a home, and have a wonderful, loving connection with my stepson. Him loving Barnacular Spectacular has nothing to do with my relationship with him.

Likewise, my biological son (who is 2) is the light of my life. When he and I are together, he is my best buddy. My parents take care of him while my wife and I are at work. If he's been around my dad for the duration of a work week, and we have some kind of family function to attend, nine times out of ten, he's going to cling to my dad and sit next with him. It doesn't mean my father has usurped my place. The kid doesn't think in those terms. He's responding to familiarity and impulse.

I guess all I'm saying is that kids are both inscrutable and do not give out love and affection the same way adults do. They respond to biology, familiarity, instinct, impulse, and the state of the moment without regard to any perception of their behavior or the way it might affect those around them. Hell, little kids don't even have empathy or realize that there are people other than them in the universe, let alone ones that have the capacity to feel.

I'm sorry for what you're going through, and I can certainly understand the emotions. It's horrible to feel like you're being replaced. My only advice, though, is to consciously and mindfully remind yourself not to take his behavior personally. If you love your son, if you set a good example for him, if you give him affection, if you set appropriate boundaries for him, if you model good behavior, and if you're there for him during the time you have with him, he's always going to know YOU'RE the dad, and he's always going to love you.