My dad has had a removal proceeding more than 20 years ago. he never showed up to court or anything.

You should help him make a FOIA request to find out what happened. Also - and don't answer this - it depends on how long he has been out of the country and back home. And can he prove that?

Assuming you are over 21 years of age, you should be able to petition for your dad and he can obtain his immigrant visa at the US consulate of his country of nationality. An I-601 waiver is likely to be required, based on whatever reason he was in immigration court deportation proceedings to begin with. He needs to file a FOIA application for a copy of his complete file with EOIR, as well as USCIS.