Is your mother his daughter? If so, then I'd say he's probably going through a lot, which may cause him to be extra insensitive.

Given the nature of your relationship with your grandfather, it may be a difficult conversation but I'd suggest bringing this up to him. Tell him if you talk about it, he'd feel less like being critical and you'd feel more emotionally stable and so more likely to be healthier.

Tragedy either brings people together or tears them apart. Let your mother's death bring about the former.