I know that kids at this age can be "curious" but I would definitely be concerned. As the babysitter I would keep track of the boys behavior and also bring it to the boys Mothers attention. There could be similar issues at home...but I would be careful as to how you approach it because the last thing you want to do is have the parents question you. It sounds to me that the boy might have someone in his life "teaching" him these things (not you but possibly a male figure). He might be the victim of sexual abuse.
totally normal for kids that age to be interested, curious, attending to… their genitalia. Touching each & themselves… showing it, etc… is to be expected. No worries ;)