The fact that the boy's mother doesn't want to "pay attention" to her son's behavior and the fact that you are frustrated by the wall she's put up (GRRRRRR!) tells me that there is some tension in this family. Often children are like the canaries in the coal mine. The child has been through a lot of transitions and loss in his short life. I'm not a therapist but it certainly sounds like he is anxious and his habit of picking the skin on his fingers may be some type of soothing behavior. Your concern is well placed and not because it "looks gross" but because this child is obviously having trouble dealing with whatever is going on with the adults in his life.  If you want to do the right thing for this little boy, I'd strongly suggest that you and his mother get some counseling. Even if you and Mom aren't married any more, you still need to be on the same page when it comes to co-parenting this child. When the two of you make the peace, he is less likely to be so anxious.