Oh Micky I feel your pain. Alexzander is a little bit older than your DS, but we have a similar issue. We had around 5 months of reasonable sleep from 18-23 months then it fell apart again. I get up at least 4 times a night, sometimes heaps heaps more. The only difference is that he calls out "muuummeeeeeee" and gets louder if I ignore him, then he starts crying.

I wonder though how your son is when he wakes up? You say he is screaming.... Like scared screaming or just crying? Does he respond to you when you talk to him or does he still seem sort of asleep? When you cuddle him, does it calm him down? Just trying to get a picture as there may be something else there rather than just bad sleeping.