Instead of opening his door are you able to leave it shut? Can he open it himself in the dark?
Is his room warm enough, he might be getting cold and waking and wanting to come find you?
Could you use one of those dummy clips for the 2nd dummy?

DD1 is in a bed and we shut her door completely and don't open it at all, she has no night light or anything but seems to be able to find the door in the dark and let herself out if she needs to.