Need help finding my fathers lost stocks. Any advice?

Hey Reddit people, i could use some advice or even some information on how to locate some stocks that was purchased through an online brokerage site and then forgotten about. Long story short, between the year 2000 and 2005 my father purchased a bunch of America West Airlines stock. He lost the browsers bookmark to the site he used to make the purchase, and lost any information he printed up on the purchases. He cant remember the brokerage firms name, website address, or any information on where those stocks are held. He's an older gentleman so his fluency with technology isn't so great. As you know America West Airlines merged with US Airways in 2007, then merged with American Airlines in 2015. Would anyone happen to know how one would go about finding these stocks, even with the company going through multiple mergers? Is it possible? We've tried our states unclaimed property website with no luck. Any help, advice, or tips would be greatly appreciated.