So terribly sorry to hear of Sweetie Boy's passing, and I do vaguely remember your post and wondered how you were faring. If it is any consolation, most everyone here on TB have experienced some form of loss or passing and personal pain associated with it.
It is never easy to lose a beloved pet, and it is at times especially hard for those in their elder years. There are aspects of mourning that do take time and certainly therapy is always an option as sometimes some pet owners find it harder to move beyond a pet's death. Sometimes, and maybe not in the case of your father, however, individuals may project patterns of mourning unresolved to a pet's death. In other words, he may be mourning more than one loss or experience of loss.
In any case, therapy is not for everyone, but I know what helped me was coming to TB and just immersing myself in stories of loss and looking at others happy budgies to help me reconnect with the memories and wonderment of my little Nutmeg and Luna who also passed in January. If your dad is so inclined and is on the internet or slightly internet savvy, maybe he will want to just browse the site to help with his loss. Another site that I found helpful and allowed me to just talk about my Nutmeg's death was this site. It's a blog created for people to talk about their pet's passing and it is a wonderful idea in how they created it:
How To Deal With The Death Of Your Pet Bird | TheTameParrot.com - Let's Try To Solve Your Parrot Problems
Or, maybe you are willing to share some of these sites with your dad where you can both have time together to browse a bit and help him to reconnect with cherished memories of Sweetie Boy. There are also a few pet loss and grieving websites, bird specific, on the internet that might help by just adding a picture and obituary in perpetuity, that can also help with grieving.
Hopefully this may be helpful to you and your dad, but in the meantime, my deepest sympathies to you and your father.