I recalled Ninja Scroll as well.
Get the PC to make a solid adamentium [to prevent sundering and because it would just look cool] double sword (including haft) and enhance both blades with returning and throwing. Have the player throw the double bladed sword giving him 2 attacks (one for each blade) against any two opponents in a circular path (treating returning as a boomerang effect). This would be a house rule for sure but does not seem to mechanically different than similarly enhancing two longswords and in fact is more limiting (circular path). Alternatively, I would allow one side to be enhanced without the other side and only allowing the one side to be used for the thrown attacks (only one attack instead of two). If it were me I would take ranged shot and also put a distance enhancement (which should qualify since the weapon can now be used at range) to triple the range. Yes this is completely plagerized from Ninja Scroll but seems to be following the character's lead.
Warrior classes excel at multiclassing because of the constant BAB. See what he wants with his character concept. Suggest he pick up a level of Barbarian if it is within his character concept, or any of the appropriate PrC's (to his character concept), otherwise just stick with Fighter for the feats. If you are allowing non-standard classes, suggest the Ninja from Rokugan.
It sounds like he is the only one in the group who can wear heavy armor without penalty. Suggest/offer some really good heavy armor. With enhancements and buffs he should have the best AC in the game. With his +3 Dex, you could offer a mithril version so that he could still take advantage of his Dex bonus to armor. However, even if it is normal steel, he will still net a better AC than anyone else as the max total AC from armor and dex for all armors is 8 and platemale is 9. Another option would be to use a buckler and shield expert (not sure if he qualifies or if it is legal). Once he has the kick arse armor spike and enchant the spikes to +5 defending (the spikes are always threatening so should be able to be used defending all the time. Moreover, they serve as a very useful defense against being grappled (where this character would seem to lose all of his power).
I would second the TWF feat chains as well as improved critical. In fact, I should have said to get each side enchanted with keen as well. That is a must.
He has rapid fire but is not in a position to readily receive quickdraw (if his next two feats are ITWF and IC). Get a glove storing or Haversack to retrieve missile weapons as a free action (if he wants to full attack while missile attacking). IDHMBWM so it may be the case that returning is a free action in which case by the rules he could just throw his sword with all of the iterive attacks. But, I like my description better

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Have him take full advantage of his greater number of attacks. A often overlooked ability is to use rapid shot in conjunction with TWF. He will take a -4 to all attacks so it is not for every situation, but in the cases where he is fairly sure he will hit, it is really useful. He can take his full number of attacks with his double sword in melee then take a five foot step back and throw his sword - a pretty nice combo if you asked me.
Another often overlooked combo is to bullrush opponents through threatened areas of your companions. If your PC is interested in more of a team effort, pick up IBR. This in conjunction with Dodge and Mobility (yes this would not be until 12th level for all 3 additional feats) can be quite nasty, ESPECIALLY with a rogue in the party. Moreover, the party could learn to incorporate this tactic as a matter of routine with T'sai, finishing off the wounded opponent once he has been pushed through the meatgrinder so to speak.
If your campaign is conducive to grappling monsters see the above recommendation as well as recommend CQC.
If you do not have the appropriate magic users in your party, I would highly recommend Blind fight and Blind sight. If the wis score is prohibitive (even after buffs) see if he can get blindsight made permanent on him. As a human he is already disadvantaged in poor lighting conditions relative to the elf and half-orc.
As he is unlikely to ever find a better double sword than what he's got, Sunder may not be a bad option either. There is also the improved sunder in S&F splat book which makes this tactic very effective.
In general, this character ought to be very effective in multiple situations with his feat selction. He has great flexibility and ought not be better than another character is in that character's specialization. However, there are ways to add flavor which should satisfy the player. The most important thing is his attack bonus. With his increased number of attacks and power attacking the drawback is the attack bonus. This player should try and increase his attack bonus in every way possible. If you allow Rokugan stuff, with his high dex he should consider II and the crane technique feat that gives a +2 to attack with a chosen weapon (with weapon focus) if you beat the opponents initiative. This with either blooded or thug from FRCS should be a nice hefty bonus. Also, this makes the combat reflexes and expert tac. chain much more attractive. If he picks up Expert Tac. he should definitely take a few levels of Ninja and may even want to consider a few levels of rogue despite the BAB decrease. Otherwise, always stick with classes that give a +1 BAB/level.
Hope that helps.