There are many schools of thought on how to react to this. My own is the following:
downplay it. Many people would take their child immediately to a specialist who only does therapy with a child that has been or might have molested. I believe that the therapy can increase feelings of guilt and shame rather than alleviate it in a child that has not had a truly traumatic experience. Sometimes of course therapy is desperately needed to help an abused child get back their sense of control, worthiness and sexual comfort. But in this case, I think a minimalist approach is best. I would not call the person a molester in front of your child. Just a person you do not trust and do not want him around. It sounds like the touch has not been too intrusive and your three year old can probably forget about this over a period of time if the incident is not reinforced by your mention of it. Kids will do some sexual exploration on their own, and two and three year olds are naturally interesting in things having to do with defecation. So I wouldn't worry too much about that except to show him proper boundaries ( ie - you don't put your finger in anyone's anus, or poke them with an object etc. ) You may need to reinforce the idea of personal space and privacy if his own space and privacy has been violated.
On the other hand, I think you might need some therapy about your child. If you think he has been hurt or scarred, you may be treating him differently and inadvertently be giving him the feeling that he has been hurt or damaged or something is wrong with him. You need to relieve your anxiety so you don't transfer it to him. You have every right to be shocked and upset if your child has been inappropriately touched, but I think you need to talk to a therapist about it so that you can understand your feelings, feel like you know what to do, and keep your child from being over concerned.
I hope this has been helpful you. I appreciate you writing and I know you will be able to resolve your fears and get back to normal sleeping and working once you have seen a good counselor.
Sincerely, Pepper Schwartz, Ph.D.
Im not a Dr, but after growing up with someone who's molestation was "played down" to protect her, I can only say that it did anything but. I think you and your son should seek some kind of councelling to help work through and deal with the matter.