Submachine, listen to Parabola.

It's pretty clear Maynard is not a stone-cold atheist.
He also has said that his anger is directed more towards the 'middle-men' of religion, that use it to forward their own ego-driven ends.

Perhaps Maynard believes that when we die our being/soul/callitwhatyouwill transcends this 5-sense existence and can experience what it thought it would experience.

As a result he doesn't 'not believe' that his mother is going to heaven, leaving you without an argument.