Apple Airpods Pro 2 have been able to function as hearing aids for a while but we all seem to be waiting for an update from Apple as they have just received permission from the FDA in the US to sell as over the counter hearing aids. I am using them as I have mild to moderate hearing loss and they work well for me.
If your dad’s hearing loss is mild to moderate it is possible to buy an older iPhone much, much cheaper than a new one to set the Airpods up. They do work without the phone once they are set up.
The main downside to the Airpods as hearing aids is battery life. I managed to get 7.5 hours out of mine a couple of days ago before I had to recharge but Apple says it’s typically six hours. They charge to full in under and hour. My plan is to buy a second pair when I start to use them all the time. Still a lot cheaper than normal hearing aids.
As others have said who have far more experience than me, without knowing the problem your dad has and how bad his hearing is we can’t help you that much.