I too live in Manhattan and know the area you're talking about. I certainly plan to let my kid take advantage of "out lunch" in middle school and so consider myself not totally anti-free range. But no, I wouldn't let him do it alone. I maybe maybe could have been persuaded if you'd said he was walking with a small group, but alone he is a target. Even if nothing nefarious happens, I think it's likely you will experience at least one phone call from either the police or a concerned old biddy making sure you know about this. (If I'm perfectly honest, I could see myself as a future concerned old biddy.) Also, while I think actual abduction or harm is unlikely, I do think any encounters with teenagers could easily go awry. I understand why you are tempted - Manhattan seems so safe now, but I really think that is partially because we adults are simply more street smart.
Could you afford a cab? Better still, a car service with regular drivers? Even getting on a bus is a better idea.