Well, he served 20 years of active duty, which means that he would have received five awards of this medal(unless he lost one or more due to a Mast/Courts Martial). Five awards would be reflected as four bronze stars on the ribbon.
What makes this confusing is that in 1963, the medal was changed to every four years, versus three. As a side note, the award frequency was changed again to every three years in 1996.
Anyway, based upon the period award criteria, he would have received:
Medal in 1951, repeats in 54, 57, 61. Then the four-year rule started in 1963- so his next medal would have been 1965, with his retirement in 1968. That makes the medal plus four stars. He missed his silver star by one year.