I think the service dates you quoted are incorrect. The Korean War was June 1950- July 1953.

He was a Private First Class (service number 1280534) in the Marine Corps with service from 29 Jan 1952- 14 Jan. 1954.

After Basic Training, according to the muster rolls he was assigned to "Service Battery, 4th Battalion, 12th Marine Regiment,3rd Marine Division".

Here's the link to order service records, there was a large fire in 1973, but luckily your grandfather was in the marines and thier records survived the fire.