The IRS is slamming my dad with taxes …………..

All of a suden my dad is being hit with back taxes from early 1990's and they are astronomical 10,000 and many others like it. Like 20 envelopes. What the hell is the problem. Now I know they are totally wrong but how do I begin to help him. Is there a specail letter to stop the fraudulent opperations of the bogus claims? Or watch my dad suffer? There must be a remmedy to the madness.