B.R.
I'm so sorry for your loss!
When I was 2 months pregnant with our first son my cousin was killed in a car accident. We were going to name him Chase Christopher until the day Eliot died. Eliot was 6 weeks younger than I and we pretty much grew up together. When I was about 6 or 7 months pregnant I wrote my aunt a letter explaining that because Eliot meant so much to me and was such a huge part of my life I would love to use his name as a middle name for our son as a tribute but only if she would be ok with that. I waited for her permission and she did give it to us...so now our almost 14 year old son has the name Chase Eliot and actually looks a bit like my cousin.
It might be easier for your aunt if you write a letter so she doesn't have to decide right away if you were to have a face to face or phone conversation. You might want to ask her a few months before your son is due to give your aunt time to get used to the idea. If your aunt is not ok with it, that's ok too. Maybe you could plant a tree in his memory as a tribute which is something else we did for my cousin.