With both of you probably turned on and anticipating this first real play session, it doesn't sound like such a great idea that he'd begin by slapping you hard with the intention of checking your limits: it would have felt a bit like having a bucket of cold water tossed over your head. Surely it would have felt better if he'd given you pain and a feel of helplessness in a way that he knew you'd be intrigued and hot by?

Sometimes the feeling of difficulty isn't just about the pain. I can take a good whipping but guess I'd react very badly if someone (a Dominant) spit me straight in the face, rubbed potato peelings and food trash at my skin or pinched my nose (and withour prior warning or play context, just improvised). Those are kind of hard limits, or rather, things I feel no turn-on from at all. Is your bf much more experienced within BDSM than you are? has he had other subs before - or is he new to it as well? If he has read about bdsm or seen movies, but never actually practised it before, then maybe he had scene ideas he felt very hot about (abduction?) and wanted to try out, but he'd felt a need to check that you could take it physically first.

It's really a sign of care that the Dom/me urges the sub/slave to use the safeword and not to be ashamed about it, but opening like this seems to have been an unhappy choice. Tell him that you need the turn-on from the start, not just the feeling of being curbed and abused.