Personality changes in dad post retirement

So this will be a long post but here it goes. Any advice welcome cause I’m driving myself to severe anxiety. My dad is 65, almost 66, and retired in June of this year after lengthy career as a behavioral specialist in education systems. He’s been with my mom since they were 23. They have had a great relationship and he’s been an amazing father, truly my best friend. I adored him. He had a hip replacement in July. I had my first child in March of this year. I live 16 hr drive away and my sister has high functioning autism and lives about 6 hrs away. They live in a very rural community, closest Walmart an hour away. Well I saw my dad last Christmas and everything was normal. I saw him in June and noticed something slightly off but didn’t think much of it. I progressively noticed something off talking to him but didn’t mention it to anyone except my husband. Eventually I told my mom and she confirmed he had been using THC gummies since right after his retirement, while he was out visiting me even. Progressively over the months his behavior has gotten worse and worse. He was frugal his whole life, but now is a huge spender, frequently making large purchases without consulting my mom. He maxed out a credit card they had. My mom has brought up his spending and he gets mad at her. He has a new found interest in religion when he has not been religious his whole life, even emailing big wig religious people. He has now obsessed with politics as well. Even going to political rallies when he was never political before. The worst is how he is acting towards my mom. He flies off the handle and gets mad at her at the drop of a hat and yells at her. I tried to have her suggest marriage counseling and he got very angry. I feel bad she’s isolated in a rural area with him. It’s giving her severe anxiety, as is me but I’m sure hers is a million times worse. She’s even made comments about leaving. He recently drove off with the gas pump still attached and got pulled over and started yelling at the state patrol with my mom in the car. He even threatened to call the sheriff on him. His behavior is just NOT him. He is a very kind, respected man at baseline. It turns out he’s been using THC products HEAVILY. My mom confirmed he smokes some pot in his 20s but hasn’t since. He’s been doing gummies, drinking THC sodas and smoking pot. He told my mom he would quit but then just went out and bought more. He’s basically high nonstop. I’m terrified of him getting into an accident while high and hurting someone.

I’m basically at a loss for what to do. We convinced him to see his PCP in early November and according to him the pcp said “he had never been in better health and refused any blood work.” My mom doesn’t want my sister or I to confront him about the THC use for fear of how he will react to her. I miss my dad. I feel helpless. I am struggling to enjoy my son at times and feel like I’m disconnected due to how much stress I feel over this. It consumes my mom 24/7, I can’t sleep. I think about it at all hours.