It is your professional and moral responsibility to report what the child has told you to the appropriate resource in your area no matter what. Do not send the message to this child that if he tells an adult that nothing will be done-his own mother has already done this. In doing nothing, you are telling him (unintentionally) that the inappropriate touching is alright and it is not. Do not let this child be a victim any longer.
You are a mandatory reporter because you are in the child care field. You need to advocate for this child immediately. The consequence for inaction on your part is too great a price this little one will have to pay.
You can anonymously call child protective services although it may be helpful to say that you are the daycare provider and what the little boy told you. And the police, they can make a report at least and it will be on file if in fact the CPS do take further steps in the situation.
Your son is pretty young, 3 years old, I would give it a few days and see how he acts, if he starts to act differently. I would suggest that perhaps he's young enough to forget, but the nature of it might not be so easy to forget. Taking him to a counselor wouldn't be a bad idea at all. I'm sure you have already tried to tell him about his privates and that no one touches. Sounds like there is a line of abuse going on... because the other boy was aggressive with pulling hair and making your son pull his pants down... makes it sound like he wasn't just curious. Curiosity doesn't cause little ones to be mean and aggressive, they just kind of check things out. Perhaps someone is abusing the 8 yr old.
Make the call. It's your duty to report abuse. You are just as much an authoritative figure as a teacher is and they have to report any kind of abuse. I hope you won't be too scared to do so, if you are afraid, keep coming back here... I'm sure I won't be the only one to encourage you to do the right thing.