This may sound stupidly simple, but I think it’s all you need to hear. You just tell him. You don’t need safewords, or to make a big drama of it. Stuff like this is simply solved by good communication.
‘So look, I have to tell you something I’ve been worrying about…’ is a great way to start. If he doesn’t stop everything, give you all his attention and take what you say seriously then he’s on his way to a quick fail for his boyfriend and dom exams.
What’s likely happening is he’s mixing reality and fantasy and doesn’t even mean it, just saying it to thrill you. I do this all the time and it’s a great way to mind fuck your sub. But it only works if you feel confident to feed that back to him.
If you’re worrying about him getting angry then give him a chance to prove you wrong. But if he does get angry or stupid or ignores you we are hitting a big red flag for him domming you I’m afraid. At the very least you need to be pulling back from the submitting side of things until he matures enough to handle that, worst case scenario is he’s not even mature enough for a relationship yet.
However, let’s not prejudge anything, be brave and say how you’re feeling, you can’t expect him to know, he’s not psychic and it’s especially hard long distance. Then see how he reacts and go from there.
The dynamic of a D/s relationship can mean there are times where you feel overwhelmed but it should only be in the context of knowing he’s going to catch you and look after you. It should be very temporary and still feel safe. If you’re feeling bad about it then let’s hope it’s just better communication needed, but if you do communicate and it’s not fixed then something is broken, and it’s NOT YOU. And you can start thinking about some of the other options I’ve discussed above.
I hope that helps, feel free to come back for more advice.