Records can tell you a lot . . . IF you can locate the right ones and IF you can interpret them. Your Granddad's personal service record was probably destroyed in the fire at the St. Louis Records Center in 1973 but there's an abundance of secondary sources from which a record of his service can be reconstructed. You can write the center anyway to see what, if anything, they have on him. You might get lucky. Let me know what they send you and when, then we'll go from there.
Technician Fifth Grade was an enlisted specialist rating entitled to the base pay of a corporal; indicates he probably served in a service or support role.