M.P.
I suggest he's anxious about not wetting even tho he's in a pull up. Perhaps spending a few minutes talking with him about how it's not only OK but also expected that he pee in his pull ups might help. Repeat to him that he's OK and wetting his pull ups is OK.
In addition you could go back in, reassure him but not take him to the toilet, and in a matter fact tone of voice tell him it's night time and time to go to sleep and then leave the room. Do this each and every time he screams, gradually lengthening the amount of time before you go in.
Take a look at your daytime potty training. Are you managing it in a calm and matter of fact way or has emotion gotten to be a part of it? If so, work on bringing the energy level down to the point that using the potty is good by not a big deal. Give him praise. Tell him you're proud of him. But also treat him in a way that tells him what he's doing in the daytime is normal but it doesn't have to continue thru the night.