No he wasn't. And the main reason for the problem was that about 13 years prior to the meltdown the government had fixed the THB\USD rate to 25\1. They controlled it by buying\selling foreign currency according to demand and supply.
George Soros realised there's an opportunity for good profit and started to attack the THB until the BOT nearly ran out of USD and has to give up.
Again, I'm not a fan of the guy, and he's done a lot of bad to Thailand, but had nothing to do with the meltdown.