how do i set up my sons ipod messaging automatically forward to my iphone

You might try Group Messaging, but for that to work you would have to create a group that included every one your son talks too... and those people would be able to see what you send to him as well.

Another down side is some other parents might not take too kindly to you "listening in" to what their child says...

Personally, and as a parent and grand parent, I think this is less about technology and more about relationships, responsibility for ones actions, and trust.... Just my $.02