Wow! What a great son!! Sounds like your Momma and Daddy brought you up well
I cant give any suggestions about the custom grips, but I love that idea!! I'm sure there's someone out there with a laser engraver that can work that up for you.
For overall "cool" factor I gotta toss in a vote for the LaserMax guide rod laser. This replaces the standard guide rod with a perfectly engineered laser. Nothing hanging off the gun, nothing to ever adjust or zero in - its engineered to be exactly on paralax and never needs adjusted.
The other thing that would be helpful would be to replace the D-Spring. This seems to be a popular upgrade.
Getting any form of night-sights on the M9 can be a challenge - - so if you were to include that, you would ratchet the "cool" factor up another few notches.
Now how about a custom holster? I'll bet you could get one of the leatherworkers over at leatherworker.net to work up a real fancy design
Of course, he's gonna need a hand-tooled belt to go with that fancy holster, oh and a hand-tooled dual-mag pouch with a matching design!
A little custom engraving on the slide to commemorate his service and I would say that is an heirloom piece that will never leave your family!!
Good luck and thank you, your Dad and your family for the sacrifices you all have made while your Father served our country!