Salaam - My brother is worried I’ll end up in Jahannam

As-salaam-u-alaikum. I’m not Muslim, though I grew up around Islam - my mum converted when I was 7. I was raised Catholic, now I’m agnostic and don’t follow Abrahamic faiths. That doesn’t mean I dislike Muslims; I just choose a different path and don’t agree with the beliefs. My younger brother was raised Muslim and is about 8. He’s on the spectrum but high-functioning. Lately he gets very anxious and upset that I’m not Muslim and worries I’ll go to Jahannam. He knows I’m not Muslim and often asks why I left and if I could come back. He even says our dad will be kinder to me if I do. He’s cried about it and seems genuinely terrified for my afterlife. I didn’t bring up religious stuff with him - I’ve kept my reasons private because it clashes with his worldview and it’s not my place to argue with him. We don’t live together anymore, but whenever I visit he brings this up and I feel terrible watching him worry. I want to comfort him in a way that makes sense within our family’s religious language, but I don’t want to lie or pretend to believe something I don’t. And from the way he understands Islam, it seems impossible to reassure him using the same theological rules he’s been taught. Has anyone been through something similar? How did you calm a young, religious sibling who fears for a non-Muslim family member’s fate? I’d like gentle, faith-respecting ways to ease his anxiety without betraying my own honesty.