The thing is, your son knows who was there for him over the years and who was not.
You cannot be removed from that past. Try not to force him to choose, or bad mouth this person. It might generate the opposite result. I guess there's also a chance that your ex, and this person, might try to buy his affections, or be the 'fun-time party-place'. That would be hard for any young person to resist. If you've raised him with a good moral and responsible core, that won't work for long.