Congratulations, you have found out why there's a D in "CoDzilla."
Seriously, druids are one of the most powerful classes on the block. You should be happy that your player was a fool that diluted his spellcasting progression with all those MoMF levels. There are animals out there that deliver more abuse than the cave troll does (legendary ape, for example).
There is no easy answer. WOTC basically issued a copout, saying, "Polymorph and everything else based on it is broken, feel free to throw it out." Given that this character is based on polymorphing, that's not a great option. There is also new errata on Polymorph and Wildshape. You should read it, learn it, and use it -- it tones down some of the abuse (example, HP stay fixed regardless of new form's con).
If it is still breaking your game, you need to handle it frankly, fairly, and OOC with your player. If they need to bring in a new character, that might be the ultimate solution. Its better to have a clean break and bring in a new (potent) character than to argue every session over polymorph errata.