Costco generally has good service and good equipment. If it was my dad, I would start with a checkup at an ENT doctor (Ear, Nose, and Throat), also called an otolaryngologist, just to make sure there are no correctable health issues affecting his hearing. The ENT may have an audiologist on staff who does hearing tests and you can get the test done there, or go to Costco or another audiologist. You don’t have to buy hearing aids where you have the test done. You can ask for a copy of the test and go somewhere else.
Then, once you buy hearing aids, they may need to be adjusted a few times, and it will take some time for your father to get used to them and for the brain to adjust. The professional you use is a very important part of this process. Also, there is a return period and if the hearing aids are not getting adjusted well, you can return them and try another brand or another professional. Make sure you know all the terms of the return policy.
Please let us know how things are going during the process.