His rate would have been Seaman 1st class, the equivalent of todays SN ( E-3).
The cap would have been worn by someone assigned to the 'Shore Establishment' of the CG during the WW 2 era. That division was involved in port security, pier guards, and similar duties. The uniform was of a cotton khaki colored material and was a coat, of nearly the same style as the current service dress blue uniform, trousers were of similar style, a khaki shirt and black neck tie . Worn by members up to petty officer 1st class. For collectors, that looks in great condition. Good find..