It sounds like a horrible situation to be in and my thoughts with you and your dad. It sounds like he is using alcohol as a coping mechanism with both of your loss but unfortunately alcohol is a depressant and makes the situation worse.
Have you tried talking to him to explain how it makes you feel on his sober days? He needs to find some hobbies that don’t involve drinking. If he’s now retired and widowed, he has two massive holes in his life which would have normally taken up his time.
My dad passed away several years ago and while thank god she didnt turn to drink, my mother struggled massively and I think was at the point of going crazy. We got her out of her rut through various things in the community, art class, learning a new language, and off her own back she decided to start going to church. He needs to find a way to fill his life with other people and other things to do. Was there something he always wished he had time to do when he worked? I know it’s tougher with lockdown’s etc but where there’s a will there’s a way.
You also mention that you keep it from people but does he have any other family – Siblings etc? You shouldn’t go through it alone and also, addiction thrives on secrecy. Hope you find a way through.