you cannot restrict his sending out info on a sheet of paper.
Have a conversation and tell him of the concerns of the rest of the board.
A board member you cannot work with should be voted off the board. Campaign; you are in politics!

Does the president try to make it sound as if the emails represent the entire board, or is it clear from the context that the emails are the opinion of the president individually?

If the former, at the next board meeting you should vote to require a majority board approval of anything being sent by individual board members. If the president continues to defy the board's vote, it should be easier to remove the president or any board member who behaves badly.

If the latter, the board should inform the president that if they persist in sending out emails to shareholders that do not represent the majority of the board, the rest of the board will send out their own email expressing the views of the rest of the board.

The way we deal with individual vs board emails is, if the email would reasonably originate with an individual board member, such as the treasurer sending out something to do with co-op finances, the board approves the text and the email goes out in the name of the treasurer only.

If the email expresses a full board opinion or vote, then the email is signed by the individual writing it, and indicates board concurrence, like this:

--- Steven424, for the board.

Either way, remember that no individual board member is above the will of the board.

They send communication as if it’s just from them and not the team. This person doesn’t understand what a team is.