I am so sorry, this must be awful.
You clearly need to meet him and so you should do so, I would plan very carefully how I handled it - for your own piece of mind for hte future.
I would ask him two questions, and I'd stay very quiet and listen hard to his answers.
- What was your thought process that made it acceptable to you to hit me?
- What have your thoughts been since (forgotten all about it? Didn't think it a big deal? Horrified but hoping for it to be forgotten? etc)
Don't put words in his mouth, especially ones you'd hope to hear. No 'But didn't you feel horrified' - that's cue for panicking about to be dumped bf to nod frantically -' oh yes darling, horrified!' No point in that.
You know what you have to do though, it's clear you do. You've had enough folk on here now agree that no, a man with it in him to be violent doesn't necessarily start knocking you about after 6 months. Sometimes it takes years for the mask to start to slip.
If it makes you feel better, I had a 5 year relationship before my DH. A long time! - but it ended, it was years ago, I hardly ever think of him now. Four years - don't fall for sunk costs fallacy, don't stay with someone because it's been a fairly long time. It hasn't really, not in terms of the rest of your life. Don't put any more time into someone who is prepared to hit you.