He probably used mixers and coinjoin transactions which are going to require advanced software
Does this mean that even a coin is mixed then it still can be track? Back in the day, I know that many people use bitcoin gambling website such as primedice and justdice to launder because there are so many incoming deposit and withdrawals there and your coins will never be trackable. He could be using bitmixer and I never heard someone track a coin after it is mix by bitmixer
If someone used a gambling site to mix the coins then someone would simply need to ask the operator of the gambling site to find out where the money went. In many cases the owner would likely be willing to give up that information because that is not what their site was designed to do.
For a mixer like bitmixer or bitcoinfog, their service is specifically designed to allow people to have privacy and if they are true to their word, will not reveal where a certain customer's money was sent to.
It is possible to track funds through a mixer. All you would have to do is know which txid that funds were sent to the mixer, the fee the mixer charges then look at all the transactions that were sent by the mixer over the next 12 hours (for bitmixer) that add up to the total amount mixed (minus the fee). For smaller amounts there will be a large number of possible outputs that belong to a certain customer, however when you have 2,000+ BTC total output, the total number of transactions is much smaller. The difficult part is trying to figure out which transactions belong to which mixers as if they do a good job their various addresses will not be spend linked together.