First of all, it's possible he might have clicked the Friend request button on accident. I would follow up and ask him if he really meant to "Friend" you. I've been told that the Mobile version of FB has the Friend Request button very close to the profile icon. I've had several friend requests from people I don't know well at all but they saw my profile pic and clicked it on accident.

So I would ask him if he meant to do so.
If he says, yes, then Red Flag. YES. That is inappropriate. Clearly you feel somewhat uncomfortable about and that's for good reason.

He can't and shouldn't try to be your "friend." There are reasons for those real-life boundaries. Allowing clients to text and email is not the same as requesting to view their Facebook page. Your relationship should exist in the office, on YOUR terms. This is to provide a safe construct for you to work out your stuff. Period.

Can I ask if you have ever felt any other inappropriate vibes or had any conversations about boundaries before?