I would leave it alone they'll catch up with him sooner or later. They will take him and probably some others to jail.... Then maybe come forward as a witness, but leave it alone for now. Besides there is no sense worrying about someone else and what they are doing when you have other things in your life that are probably more important.
You should contact with Income Tax Department in your area. An income tax officer will investigate the case and if your boss is guilty, appropriate action will be taken against him.
Are you certain of all the facts you have cited? I am not defending any illegal tax activities but make sure you are not making erroneous accusations which could result in action against you.