My dad is losing his mind

I'm 18 and have a 15 year old brother with autism. Our mom died 2 years ago and it's been really hard on everyone. My brither started wetting his bed and my dad completely stopped showing any type of affection towards us. He said we reminded him of mom too much. So he just goes on about his day like we dont exist. Lately he's been slipping mentally. Saying how mom is buried under the ground and is dying and needs our help. I told my dad to see a therapist but he just got mad. Last night he completely lost it md started digging outside in the rain to bring mom back. I tried to get him inside but he says he has to save mom. I finally m de him comw unside. He's outside again digging to get mom out and terrifying my brother.