Co-authorship just because a Professor provides me the employment?
This is a form of gift authorship. The worst form, if he imposes it on you. I know some people would think otherwise, but my last supervisor, who was a big name in his field never signed papers he had no contribution on. And this was despite of the fact that he was the big grant writer and winner of the group. 

It's hard to tell what to do. In my case, I used to simply do what I was told by the boss. Only I never had such dilemmas. But, I had a few friends who were in the same situation as you. When they refused co-authorship, their supervisors got angry. I work with one of these supervisors at this time. The guy went around and told everyone my friend "stole" his idea. Nevermind that dude had no ideas to begin with. The other guy, wrote a negative recommendation letter for my other friend who lost a very tempting postdoc offer over it. 

My recommendation is to stall until you leave the group. Until then you fix all the details you need to fix for the paper and find out if your boss plans to contribute anything else to it. If it wasn't his idea and had so little involvement in it, once you are gone, you can publish without permission. As a courtesy, you can tell your superviser, after you're gone, that you are thinking of including him in the paper if he has time to make a contribution you think would be useful and within his capabilities. If not, include him in your Acknowledgements  section.

Edit: backup your correspondence with him (emails). You may need them if he's the kind of guy who gets upset and writes your editor that you "stole" ideas from him. Hopefully, it won't be the case.