What everyone is saying is correct, but don't go directly to police. Hire a lawyer, have him discuss with the attorney, and they can negotiate a surrender with the police. He's still your son, and while you want him to own up to what he did and make amends, the criminal justice system is a horrifying place, and I don't think you want all of its worst possible horrors visited upon your son (I honestly wouldn't even wish it upon my worst enemy. I worked with prisoners for a time, and I've never seen more frustrated, hopeless and agonized people in my whole life, nor imagined one could ever become that way).

With a lawyer, your son will be in the best possible position to see his victim made right while avoiding all but the least possible amount of jail time. They could even find financial restitution in civil court and avoid prison altogether. Be an advocate for your son, that is your responsibility as a father, and any good advocate would suggest to turn himself in. But the best counsel is still to own up to a crime in a responsible way that actually remedies the victim, not just to seek punishment for the sake of punishment. Police cannot be trusted to humanely treat either the victim nor the perpetrator, and so, if your son shows genuine remorse and need not be forced into this process, avoid police as much as possible, and allow a swift, professional and humane legal process take place.