Re: 161st Infantry Regiment, 25th Infantry Division WWII Philippines
The 25th Infantry Division has a web page online for their association which I would suggest you get in touch with.
You may also request your grandfather's Individual Deceased Personal File from the government. When you send your request to the below address cite the Freedom on Information Act and your relationship.
Total Army Personnel Command
ATTN: TAPC-ALP-A (FOIA)
DCS Personnel and Logistics
200 Stovall Street
Alexandria, VA 22332
Be forewarned that these records can be very graphic.The next web site will give you some history of the division.
Good luck to you in your research.