It is valid to make a bequest to both an heir and a non-heir according to Law no. 71 for the year 1946 CE of the Egyptian bequest law that is itself based on the opinion of some scholars. The bequest is executed from up to one third of the estate without the need to obtain the heirs' consent. Anything in surplus of this requires their consent—if they agree, the will is executed; otherwise it is not. But if only some of the heirs agree, the will is executed from the share of those who consented to the surplus and not from the share of those who did not. This is only applicable if the testator was of sane mind and understood the nature and effect of his testamentary act.

The ruling
Since the husband's bequest was verbal and no one heard him make it except for his son, then the ruling for it depends on whether the heirs believe it. This is because they are the ones to decide whether it is to be implemented. If the heirs believe the son, then it is obligatory to execute the bequest from one third of the estate and the heirs' consent is not necessary. However, as mentioned above, consent must be obtained for anything in surplus of one third of the estate. If, on the other hand, the heirs do not believe the son, then it is not obligatory to execute the bequest and its implementation is voluntary.
Allah the Almighty knows best.