Hi thanks for writing JA
I have read your post and understand why you might be disturbed. It does happen though that lots of kids, especially very bright and curious kids do behave this way and it is not an indicator of any kind of abuse. It does not necessarily mean abuse at all but it is disturbing because we consider this socially inappropriate behavior. I actually have a handful of children in my practice right now who are sexually stimulating in socially inappropriate ways that have no trauma or abuse history and I wish I could tell you that he will out grow it, but he might not.
What I tell my parents is to give the behavior three months and if they see no improvement to get the child to a child specialist. I am sure that does not give you a great deal of comfort but no in and of itself, anal stimulation in a three year old is not indicative of any mental health issue or deviant behavior. I hate to say this so casually or simply to you, but he probably likes the sensation so he keeps doing it. I so know that is not what you want to hear. I just so know that, but it is the case with a lot of children and even adults and again no, it does not mean he is deviant or homosexual or anything.
I would keep doing exactly what you are doing and see what happens. If after three months he is still doing this? Seek some professional behavior.
I can offer one other piece of advice here, but it has only worked one time for me in over three decades?
I ask children why? and one time in thirty years a child told me? Otherwise kids know somehow its bad and they are not forthcoming.
As long as he is not causing himself harm physically there is nothing immediate to consider but if it persists? yes, surely get a therapist.
Remember one thing please? You are I are adults so we have a lifetime of conditioning and training to know that inserting things into various orifices is not okay. Young children especially toddlers are naturally curious and do not have the values and mores that we do. It is most likely less a big deal for him than it is for you and I?
Three months on this?
Let me know if we can help you further and you have more questions?