Another alternative is to ask for a unpaid leave of absence. That will give them time to reconsider and you time to think and do.
Or, take your vacation.
If he does this, there's a chance of him getting fired. It doesn't send a good signal.
He is taking a risk of getting fired already. Depending on whether the boss likes surprises and/or if you want to not blow up your bridges, asking for a leave of absence is a subtle way of giving notice before you give two weeks notice e.g. after they come back with - 'Sorry, we can't do it' - 'Thank you for taking the time. I've thought about this a long time. I'd like to tender my resignation'.