Experience used to say 7" blade. Experience now says 5" blade, 4" if you're really good, 6" if you really need more. So that puts us squarely in BATAC range. If he thinks he'll need more than that, go with the SJT or JH. All these knives are wicked on both ends, what a Marine needs.
But I'd really push for the BATAC: to put it another way, it's small enough to carry instead of being big enough to leave behind. Marines walk a lot, carrying a lot.
But above I'm really talking about a Combat Knife. Something slightly different than the Ka-Bar he might otherwise carry and even a bit more capable.
Another knife to think about is an EDC knife, like Horton brings up. More often than not Soldiers and Marines are leaving their combat knives behind. If you get him a Public/Desert Defender or another medium/small blade like that he may be much more likely to actually carry the thing. Counter-intuitively, giving him a "lesser" knife may mean he has a knife when he needs it, so much better than an ASH in his assault-pack in the back of the Humvee or MRAP.
Best of both worlds he'd have a PD on his ballistic vest (those things weigh 65-75lbs when the mags are full, and upwards of 80 if he's carrying a camelbak too) and a Skinny ASH in his assault pack. But with the BATAC it's possible he may never need anything else.
Thinking about all that weight might help your decision.