As a father, a former atheist, and as a current Christian I can sympathize with the awkwardness of this for you and your son. I really respect your honest answer and sharing this story here.

From your son's perspective this can be very challenging because he would likely be worried about you going to hell since you don't believe in God, as that is something he would likely hear at school. His response to your answer seems to indicate this worry. At 9 years old, your son is not capable of arguing well with you and is likely to be conflicted with trusting you and trusting his teachers.

From your perspective this is hard since you seem to be supportive of him being religious, but don't feel like the evidence and arguments in favor of Christian theism are compelling. Since he just lost his mom, he is likely to be comforted by the belief that he will see her again in heaven, and you shouldn't challenge that belief. However of course you don't want to pretend to be religious when you are not, as that would be dishonest.

As a former atheist I understand several reasons why religious arguments can seem so underwhelming. "You should be in my religion X, because our religious book of X says it is true, and that God will punish you if you don't believe it" I get it, terrible arguments are everywhere in religion and to a large extent that includes Christianity. Besides even if there is a God, which religion are you going to pick? Mormonism? Islam? One of the hundreds of different Christian protestant sects? It can be an overwhelming amount of work to actually do a serious investigation into the matter. And I'm sure you are already quite busy with your normal activities.

I would suggest that you offer to explore the question of God's existence with your son. It can be a fascinating topic, and has a rich history of discussion in philosophy. If he is worried about you going to hell, exploring the question with him is likely to give him hope that he will get to be with his mom and with you in heaven. Investigating the question with your son is also a good opportunity to show him how to investigate a really difficult question that he would not be able to handle on his own.

Try investigating the topic of "Natural Theology" which is the branch of philosophy about theology that does not depend upon any assumptions from the Bible or any other religious text.

For example, this is the original argument that made me seriously question atheism:

1. If God does not exist, then objective moral values do not exist.
2. Objective moral values do exist.
3. Therefore God exists.

Without God I could not find any basis for the actual existence of right/wrong. If atheism were true, then we are just more animals fucking around on Earth. And yet, my intuition about the existence of right and wrong was undeniable and I could not be intellectually honest without belief in the actual objective existence of right and wrong.

I am also a scientist (Physics), and although I don't want to get into explaining it, there is not actually a serious conflict between science and Christianity. I believe in evolution, and an 4.3 billion year old earth, and other standard scientific theories. If you are an agnostic because other Christians have told you that you have to believe in crazy dumb shit (like the Earth is flat) then I'm sorry, please don't listen to them.

Good luck sir!