asperge'rs and sexual exploration

Hello,

My son in 15 years old. My sons mother found him in the closet with his 14 year old brother. Prior to opening the closet door his mother overheard my son asking his brother if they could kiss. When she heard this she opened the door and found that the two of them had their pants down.
My son was diagnosed with Aspergers about 4 years ago, his mental age is between 8-10 years.
This is not the first time something like this has happened. About a year ago, he had similar behavior with a neighborhood female child about the age of 4 or 5. He has also been caught trying to play exploration games with his sister who is 8.
My son lives with his mother,step father, brother and sister. I do not live with him. I get a call that my son is a freak and a pervert everytime something like this happens. I was wondering how I should deal with this situation and IS THIS BEHAVIOR NORMAL WITH ASPERGERS? I tried to explain to his mother and step father that they should not embarrass my son about this but they don't listen. They punish him, tell him he is a pervert and tell him that if he continues this type of activity, he will go to a juvenile detention home and then to jail. Please help me with a little advice or is there a book that you could recommend?
Thanks