After lengthy discussions, I'm thinking instead of some monk/druid hybrid which risks balancing issues, to just add in some Ki Powers for the player.
I have 2 Ki Powers that -mostly- simulates what he wants without requiring him to morph into Dire Tiger.
Ki Bite - Gain a Bite (1d6) Attack with a full strength bonus added to damage. Spend 1 Ki to have this effect for 1 minute per level. Must be 8th level to take this power.
Ki Jaw - Prerequisite: Ki Bite. Spend 1 from your Ki Pool to double the dice for any natural attack. This effect lasts for 1 minute per level.
Also thinking of simulating Pounce (natural attacks only) as a Ki power... It seems fair to me, basically granting an additional natural attack with his 2 claw attacks, to stack with his Flurry of Blows (albeit at -5 to attacks). And a "Strong Jaw" spell at the cost of 2 ki.
His goal is a whirlwind of death (how many attacks can you squeeze into a round). This puts him at a couple of attacks higher than Greater Two Weapon Fighting, but without the bonus Critical that Rapiers get (which I -think- balances out evenly). And while true that TWF requires 3 feats to do fully, he'll undoubtedly have to spend a lot of feats getting the Ki Pool necessary to do this multiple times a day, as well as fuel his other powers...