If you give him a new 'flurry of blows' at-will that has the requirement unarmed-strike only, it shouldn't be a problem. All the issues of stacking up magic items and lasting frost vulnerabilities and other such wierdness disapear. He gets whatever goodies from feats or whatnot to improve his unarmed strike as you care to dream up, but that shouldn't be a problem (and, if it is, you can adjust the homebrew stuff).
Twin Strike (Ranger) does show itself to be mathematically superior to Careful Strike (Ranger), and Careful Strike is identical to Sure Strike (Fighter). The impression I get of Sure Strike is that it will only be taken by Human Greatweapon Fighters who don't have any use for Tide of Iron. So, arguably, Twin Strike probably shouldn't go to a fighter. Another objection to it is that a Defender Fighter could use his shield as an improvised weapon to take advantage of it without giving up his weapon-and-shield perks. But, if, as you clearly intend to, you limit it to unarmed attacks, and keep the power of unarmed attacks in line, it shouldn't be a problem.