First, I am so sorry this is happening to your child. As an adult this can be terrifying so I cannot imagine a little one being so scared they are screaming.
Can you log the days he sleeps through the night/does not and jot a few notes? Do you know what his nightmares entail? Then perhaps you can see if there are differences, maybe that night he was on the iPad before bed instead of looking at a book, etc.
You could try something to calm him before bed like a diffuser with child safe essential oils, I have had success with ‘calm them down and sound asleep’ from eden’s garden.
I wish I could be more helpful but if the professional is saying it will pass, hopefully it will.