if you are physically assaulted by an employee that is on duty right outside the front door, can you sue the restaurant chain.

You can always sue anyone for anything, but the question will be whether the manager was acting withihn the scope of his employment when he punched your son.  If so, his employer (which may or may not be the restaurant chain; it could be the individual restaurant) would be liable to your son for any damages caused by its employee's wrongful conduct (assuming that a judge/jury agrees that the manager's conduct was not justified).  If the manager was not an employee of the chain, your son would have a much more difficult time holding the chain responsible for the manager's actions, although it is still possible that the chain could have had something to do with the manager's hiring or training which was arguably improper and contributed to causing the incident.