I recall reading an award for a Meritorious Service Medal that my father received, which is not on his DD214. How can I verify this award?

If there is a certificate with a Permanent Order (PO) number or the DA Form 638 (Army) with a PO number, it is a legitimate award. It may have been done after the DD214 was cut. You would need to go through each award and match it to the 214. In the past, successive awards of the same type were cut with language like 'Meritorious Service Medal, second award' or 'Army Commendation Medal, 3rd Oak Leaf Cluster'.

You can file for a records correction. They could issue DD215 to correct it. https://www.archives.gov/veterans/military-service-records/correct-service-records.html