I didn't think you could (without mods, I'm on the 360), but I thought I would check because I saw this article and the screenshot seems to show the PC wearing that armor. And I wants it.

The article talks about possible future DLCs if you're interested.

http://societyandreligion.com/2013/03/skyrim-dlc-bigger-dragonborn-dawnguard/

I've never been able to obtain his armor, just his axe.

if you're on PC you need to first add it with a console command, but it will be invisible in your inventory. You then need to equip it with a console command (and it will still be invisible in your inventory). It will only show for a male character, with it looking like the DB robes on a female character.