Well, my feeling is that you probably aren’t going to come out with a win here if you are that vulnerable in this area. I am not familiar with your condition, but I will say that everyone loses hair, and if this is a particular phobia for you, grounded in a real physical condition, I would recommend that you see a doctor to confirm your fears before you decide to perpetually shave your head. As such, unless your FWB is completely in the loop about this condition, his first thought is probably also that shaving your head was a bit of an overreaction.
I happen to like the shaved head look on some people, so that’s not why I say this.
In addition to all that, there’s no way he could have predicted that you would make this drastic move, and I would have been amazed if he managed to say just the right thing in response to hearing the news. It doesn’t mean he won’t be totally accepting of the new do – he might love it.
But there is a risk that he won’t like it, and you need to be prepared for that. You took control of your hair loss when you grabbed the razor. My advice is: don’t let his reaction rob you of that control. If he’s going to be critical of it, and that hurts you, tell him the two of you are done, and move on. You know very well that a shaved head is not a good enough reason to break up with someone, and you’d have lots of reasons to be pissed if this was a real relationship – but it’s not, so you have to be able to deal if he doesn’t want to stick around.
The bottom line is, you can’t guess how he’ll react when he sees it. But you should not be afraid to see anyone because of how you look. Least of all the guy you’re sleeping with.