What happens to a missionary if he choses to leave his mission early?
Part of the answer depends on the reason why the missionary leaves his mission prematurely.
Did the missionary get sent home for breaking their temple covenants? Then not only would he be sent home, but the missionary may have to go through a repentance process.
In either case, any time a missionary is sent home early, those in the congregation will talk and ask questions. Be ready for this as they’ll be naturally curious. This makes it much more difficult for a missionary who was sent home with a “dishonorable discharge” to remain active.