On another tangent - if he is moving in a few months after you do, starting a relationship with her would end up being a LDR. He would likely want to go back to visit her on weekends which would probably cause a lot of stress for your relationship, or perhaps he'll be busy and distracted texting and emailing and calling her while you end up setting up the new place all by yourself. Has he thought that through?
Dealing with a new dynamic when you are not living in the same place will also be stressful, and you have a higher chance of being plagued by insecurity, jealousy, fear - but without the ability to be face to face with him to talk things through.
Because of all the pitfalls of dating where you work, I'm not fond of my husband dating people he works with, and he's been poly for over 20 years so would hopefully avoid most of the potential problems. If he were new to it, I am pretty sure it'd be a hard limit for me, due to the potential problems with it.
In your place, considering your current limits with your OSO, I would be hard pressed to be OK with him wanting to see a coworker, but I would hope to segue that into new boundaries for both of you once you were settled into the new town. It's OK to say no to the situation.
Happiness will never come to those who fail to appreciate what they already have.