I hate the "prove me wrong" thing, but I think this is one of the very rare cases when there is a single, right answer.
"Son, did you kill that girl?"
"Yes"
"Are you 100% sure that she is dead?"
"Yes"
Find a lawyer and connect him with your son. Then immediately call law enforcement and tell them what you know.
Of course, it's a heartbreaking thing to do. But it's the only right thing in my mind.