Crystal clear,
well done for writing the letter, but in the meantime I would be looking for another job, simple as that.
By the sounds of it, it wouldn't matter if he hit you, he wont get sacked.
Value yourself more, you do a good job, been a good employee
maybe its time to look for work elsewhere maybe even part time, and with the spare hours try and get your own little bit of work.
Would be hard at first, but better than looking back in a few years wishing you had tried it, because theres nothing to say something like this wont happen again working with someone else.