It's called the MJOLNIR mark V armor. It's supposed to be a reference to old Norse Mythology.
In the newer iterations of the Halo series (i.e. 2 and 3) It's MJOLNIR mark 6 armor.
ODST armor is similar in a sense, but is not the same.
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He doesn't have one.
go to halopedia it's there
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A Spartan wearing Mjolnir armor weighs about half a ton total.
Play continuously enough to gain the ranks require to unlock the armor he wears. I believe you should have all his armor by brigadier or general [give or take a rank or two] once it is visible AND unlocked you need the amount of credits required to get that helmet to wear.