i think just the fact that you wrote about the incident says that you really don't feel comfortable with the method used...
i think since he knew from your reaction (and he must have as you turned away and he pulled you back)... he should have encouraged you or asked or reminded as someone else here said to use your safeword... he should have talked to you more during the slapping.
But also.. as Louise said, this being done early in the relationship would bother me... whether it's a viable method of teaching a sub to use a safeword or not.
And if he was using that method.. well, you didn't say you ever did use the word, he stopped when you cried- i mean, i think he should have stopped when you turned away, at least.
However, on a positive note, i find it good that he comforted you and talked to you afterwards. And as Dragon's muse said, you need to talk outside of the bedroom- long and intensely, about everything.

Good luck, dear