This sucks, I’m sorry.
I think you already know what you need to do and that’s limit your interactions with this guy. That’s how you “kill” a crush. There’s no point in putting yourself through watching the guy you have a crush on be with someone else. And, you’ve only known him for a few months. It’s not like you have to end a life long friendship here.
Also, I’d advise against telling him how you feel. Life isn’t a rom com where he will somehow boomerang around to you if only he knew. It’s just going to make things weird and awkward and then you’ll feel even worse.
So, limit your contact to work stuff only and also make a point to hang out with other friends, join a new group or even get on dating sites. Essentially fill up your life with things other than this guy. If you’ve got a full and fulfilling life, you won’t be “forever alone” even if you’re single.