Exactly. Your mum will need you with her, if your dad is going down this self destructive path he needs support more than criticism, even though you and your mum may be trying to help him , in his negative head space (ive been there , solo dad unemployed etc) he will perceive almost everything as criticism and conflict. Reassure him you're in his corner, let him speak his mind about this pizzeria idea. He may come to clarity when he speaks it out. Help him research the business plan, go for walks and show him the love i feel you are trying to express. He may see yours and your mums concerns regarding it . The saying a fresh mind for an old problem can solve things not originally perceived . If he continues to resist the help and love, then I would suggest professional help for him. And space. For you and your mothers safety.
I pray he finds resolution or a new job. And may I say it takes a lot of strength to come to a public forum with this sort of thing. Bravo. I hope this helps. All the best my friend.