dighsx, Welcome to the forum, Looks like your just getting starting. Suggest you look through the 'military Service and geneological research part of this Forum, and look at the "requesting copies of military records" thread--it gives a lot of information. The acronyms are always tricky as each branch of service has it's own. You didn't indicate whether he was Army or Marine, both were on Okinawa, but a Capt. is usually in charge of a company (about 100-120 guys. It seems logical he was the CO of an Infantry company. CO usually indicates "Commanding Officer" or Co. for company. Think you may have read the ING wrong, believe it should say INF, which stands for Infantry.
Your Grandfather was KIA and the VA should have a complete record, also NARA if they wern't burnt in the 73 fire. Your best bet is to write to,

Department of the Army
U.S. Total Army Personnel Command,
200 Stovall Street
Alexandra, VA 22332-0404

Ask for your grandfathers Individual Deceased Personnel File (IDPF)
This is a freedom of information file, so you'll have to be the next of kin, or have his next of kin sign the letter. Give them all the information you can, especially his name, rank, service number, (O-385456) and date of birth and death if possible.
This will tell you about his death and the disposition of his body and personal effects.

Also write to NARA--get a form-180 off their website and see if they have his records. In the same request, ask for a replacement set of his medals and awards.
I always send a letter with the form-180-you can ask for his awards in the letter.
There should be a lot of information out there. Check back if you need some help.