The USMC doesn't usually use any specific, colorful markings. Usually, they only have USMC and a 6 digit serial number on them. During ODS, they also had the Coalition Chevron on them for friendly ID, each side and back. These could be found any any generic number/letter decal sheet. His best bet is to find a pic of his dad's HMMWV and model what he sees.
USMC & Serial #
Still the current pattern on much more recent M1151 Armored HMMWVs.