start by having him check with his command about what's allowed for personal carry -- some commands don't allow a personal blade with more than 4" blade -- I recently found this out the hard way when a blade I gave a marine was shipped home because it exceeded his unit standards.
that said, for his MOS, serrations are a good thing.
a Molle back is a must -- again, marine regs as I was told, all personal gear has to be molle compatible. (yes, the molle locks meet the requirement, but the "jump proof" MOLLE back is that much better)
the 4S will do just about anything he needs it to in a package that won't add lots to his load out or take up a lot of space on already full LBE.
The 6s gives better grip options when wearing gloves and a longer blade for digging.
The clip point on either the 4 or 6 is handy for stabbity type actions, and also allows for an edge that can be used to cut wire without compromising the main cutting edge.
the 5s will take scary amounts of lateral stress, and is designed to cut through thin sheet metal.
sorry for the novel, hope this helps with the choice. best of luck to him and thanks for the service!