Good question. :) "he is being very formal'' = at this moment now ''he is to be very formal'' = this is expected of him (generally, a future form)

''Wow! he is being very formal today'' (Normal verb) which means, he is behaving very formally.Usually he is not formal. ''So, he is the one to be very formal tomorrow night'' means he will behaving very formally tomorrow night.

As far as I know, "To Be To" refers to an obligation which has been made by rules or something like that. It's a little similar to "To Be Supposed To"

let me give it a go first. 'he is being very formal', the time is now, means he is acting very formal right now, 'he is to be very formal', the time is future, means he is planning to act very formal in a future time.