Most Helpful Guy
I don't know if my case was as severe as this, but I've been in his place. Make it brutally clear - like you're writing a lab report clear - that you have no intention of resuming your relationship ever. He has hope - maybe you've been amicable and he's taken that as a sign; it will save you and him time and stress to crush that hope. The obsession will break when he starts looking to other women, but he won't start doing that until he's clear that you aren't an option.
Good luck. I'm really sorry that we get like this, especially early on.