In terrorem trust clause wont work when hostile son just wants to tie-up assets & deliberately inflict pain on my beneficiaries.

"In terrorem" clauses are unenforceable in some jurisdictions. If you are disinheriting this son, make sure that your will and/or trust are "updated" (simply reprinted by attorney and re-executed in 6 months or so) so that this son has been disinherited more than once in your documents. He will not have "standing" in court if he has not received an inheritance in more than one document. See your estate planning attorney for details.

I think you appreciate that there is nothing you can do to stop him from filing a legal action. The only thing you can do is position your estate the best as possible for the beneficiaries to seek a quick motion to dismiss. Atty. McMahon's suggestion of executing a new will, repeating the disinheritance, is not a bad idea. Unfortunately, what I think you might have to do is go beyond the simple disinheritance clauses we use and beyond the simple statement that his omission was "intentional and not inadvertent" and write something in the will that is a little more expansive - perhaps expressing disinheritance is stronger terms, perhaps explaining why, specifically addressing that there is no undue influence, etc. He has no right of inheritance by will in law, and only your words can take away any factual ambiguity. You can only hope that your beneficiaries can knock out any lawsuit early and quickly, and with an award of attorney's fees against him if truly frivolous. Perhaps a video recording of you making it clear your feelings. Not sure if that will be admissible later, but could be made so, and it would allow a judge to read your credibility and sincerity.

This question sounds a lot like other ones I have answered many times on Avvo. If all you want is helpful information, I would suggest that you look at my earlier answers and the answers of other lawyers to those questions.
But the primary thing you need to do is find a real lawyer to help you come up with a real plan, based on your real, and very complicated, and obviously painful situation.
Figuring out a way to keep the terrible thing you describe from happening will require an attorney who knows a lot about trusts, and a lot about probate, and a lot about people to take the
time to learn a lot about you, and your assets, and your family. Then the two of you can come up with a plan to avoid the disaster you describe. Then you will have to implement the plan, which will include training your other children how to deal with challenges from your son. Finally you will need to update the plan as things change, as they inevitably will.
I can't tell you what that plan will look like, and neither can any other lawyer who has not taken the time to understand your complicated and painful situation. You need real advice. I hope you get it.