Under the current immigration laws, you are not going to be able to legalize your father within the US.  He would have to go abroad to consular process but by leaving he would trigger an automatic 10 year bar to reentry which can only be waived by a showing of extreme hardship to a US citizen spouse/child.  this is fairly difficult to do in most cases but if you feel that he can show that, then you should consult with an immigration attorney in your area who handles these types of waivers.