His accountant can no more hand over the president’s financial records than his doctor can hand over his medical records. Executive privledge.
Maybe executive privilege can be asserted by the POTUS but the accountant's communications with a client are not typically privileged. A doctor has privileged communications and can ignore a subpoena without consequence if the patient doesn't want the information disclosed, however an accountant only has confidential communications meaning that the communications must be disclosed if under a legitimate court order, even if the client doesn't provide permission.
If the work was ordered by a Trump attorney, it might fall under the attorney's privileged communications as a work product. The Trump attorneys and the accountant's attorneys may find other reasons to challenge the legitimacy of the subpoena but it is more likely a delaying technique than something that would ultimately squash the subpoena.