Favoritism tends to be a common issue when marriages break up and new ones form. The man typically begins to feel he has done his child wrong by not making the marriage work and tends to over compensate for that reason. Unfortunately this is not helpful for the child or the new children he has.
Now the bad news is most fathers tend to be very protective in situations such as these and typically feel like the new spouse is the one in the wrong. Often they feel like the new spouse is being unfair to the child or even making differences of her own. The fact is often the father feels torn or feels like he must take one side over the other. The problem is this tends to ruin marriages and cause even more harm to the children.
The key is approaching this in a different light. Instead of jumping up and pointing out the differences take note of it and write down what happened as well as the date and time. Once you have a good amount of information sit down with your husband and have the talk.
Let him know that you know he loves both children the same as you and that you know how he feels guilt over his son having to come from a broken home. Explain to him that you are concerned about his son as well as the son you have together and that you are worried that his guilt and over compensation is having a negative effect on the children.
Point out that by showing his guilt his son is slowly becoming behind what other children typically do (This would be the wiping himself and any other issues). Let him know that you understand why he is over compensating, but that you are genuinely concerned about the 8 year old and his well being. Coming from a broken home is hard..........but if the father is playing a active role and showing his son that he can still be a father to him his son will grow stronger without the need to try and act younger for attention.
Let him know that he is getting to the point of showing favoritism and show him the list. Again let him know you are not mad.......but are very concerned about how this will effect your younger son if it continues. Explain to him that your son is still young enough that he is not hurt from this yet, but with time he will learn the score and will eventually resent him for his actions.
The key is to not make it look like you are placing demands or that you are angry.......try the "I am concerned about both children" approach and you will have a better chance of getting him to see what he is doing to the family.