How do you propose that Trump blocks Bolton from testifying, if he is subpoenaed? Executive privilege? That is a defense the President can invoke, not a weapon to stop other willing people. e.g. if the subpoenaed witness doesn’t want to testify, the president can probably use privilege to protect him/her from doing so, depending upon the nature of the intended testimony (which would possibly be a muddy area, and a dispute unto itself). But privilege can not stop a willing witness from testifying.
He could go to court and ask for a restraining order to prevent Bolton from testifying, but on what grounds? Has it ever been done before? And would a court grant such, knowing that it would be a significant upset of the checks and balances in place?
What else would you propose he do?
The most likely scenario I see would be for his legal team to simply threaten to do everything they can, raise every legal challenge they can, to prevent Bolton from testifying, and effectively threatening to drag the impeachment trial out for as long as possible (years) in the courts, in the hope that the Republicans in the Senate, not wanting this scenario, decide to reject the request for Bolton as a witness, as being the least bad case.
Time will tell.