I've had pretty bad sleep paralysis recently. I'm not an expert because I'm still trying to figure it out myself but I can tell you that it's bloody terrifying. When it happens to me, I know in the dream that I can't move, and as other people describe, I feel like I'm falling "under". As I'm waking this feeling continues, and I still feel like I can't move. You know like if you lie on your hand and it goes asleep on you?? and you have no control over it.....in a way it's kind of like that. But it happens to your entire body. I'm trying to move myself to feel things like bed clothes, locker in order to be able to reconnect with my body and therefore move and regain control of it, I need to feel some kind of sensation but often I can’t. Because I can't move while I’m waking up - it's like the nightmare carries over into my reality.
Your boyfriend is lucky to have you there. You really can't help him during the dream unless you notice him making sounds, etc but as he comes out of it - it may help to touch his arm and talk to him - especially if he struggles with reconnecting with his body on waking. For me I need to get feeling back in my body so I can move it. So touch might be key – ask him if this applies to him.
Another thing is focusing, it feels like I keep going to fall back asleep if I don't fully wake and that’s scary because it feels like I'm going to slip back into the dream and fall under again. A part of my dream often involves trying to get other people's attention to wake me up, opening my mouth but no sound is coming out. I sleep alone so when I do start to wake, I will do whatever it takes to wake myself up fully, often switching on a bright light when I can move. This might not be best but I'm trying out other things. I can’t feel sleepy, I need to feel fully switched on in order to feel safe again.
Just telling him you're there, he's safe and it’s just a dream should help. While he's making sense of where he is and connecting again, it might be really helpful to repeat this so he can hear your voice and it’s consistent. I've had a small light suggested to me, if it doesn't bother you both, or something he can see when he wakes up. If his dream is happening in your real bedroom - which is what happens to me - he needs something which differentiates the two when he wakes, so he realises it isn’t the dream continuing. Hope this helps a tiny bit. Wishing you both luck :)