I am sorry you are going through this. It must be tough on both of you.
My dad lost his mum, and it took a toll on him for years. Grief is not linear, you have good days and bad days where you miss the person more than life itself. There's denial, emptiness, sadness, anger, frustration, happiness that they are in a better place etc. Time heals but the pain never really goes away. My advice would be to just be there for him, i know it must be hard, but he is doing his best.
I would also suggest trying to see a grief counsellor which could help him understand and grieve in a more healthy way. You also do not deserve to be treated badly. But lean on the side of understanding.
I hope things improve,