There are many apps and programs in existence for this purpose.
Remember, from a legal standpoint (most jurisdictions should be very similar) have to own the phone, and your son must be under 18.
While I can at least appreciate the wanting to 'listen in' etc, the fact you want to record conversations and activities.....
I'd suggest you maybe reconsider whether you feel your son should have a cell phone in the first place.
If you feel he needs to be tracked that much, then maybe take his phone away? Assuming you own it, and he's not 18*.
*If he owns it, or he's 18+, you can;'t really do anything anyway.