Who does he live with?
He did this in front of you?
Did you say anything to him about it?
His behaviors of licking his fingers and putting his finger in his rear is learned behavior. Meaning this is behavior believe he is learning from someone else. You are in a hard position right now, you can either speak to someone at child services, or you can talk to the parents and let them know your thoughts. Although it is pretty normal for a child to play with their genitals at almost any age, this goes beyond curiosity. Have you tried talking to him and asking who showed him that or if he ever does it with anyone?
The hardest thing is to get parents to look at what might be going on. Perhaps they would consider putting hidden cameras to see whats going on while they aren't there. Or they can try and talk to the child as well and see if they can get any info out of him. Its scary to think someone could ave done something to him, but again, in my experience, this goes beyond the ordinary.
I welcome your thoughts, let me know if you want to talk more.