Is it possible for a 7 yr old to get sexually aroused for self gratification purposes?? ?

I just found out that my 6 yr old autistic son was sexually molested including penetration by a much larger 7 yr old boy. I was also informed from the boys' parents after bringing this to their attention that there has been 4 other incidences in the last 8 months where he has done this to other kids & is in counseling. I called child protective services & the filed a report w/ local police. I was informed by both the police & cps that the 7 yr old can not be criminally charged because a 7 yr old does not have the ability to become erect for sexual gratification w/ the intent to harm and therefore what has happened to my son is considered "experimentation" and that all boys do this? I'm at a loss as where to turn.