Your parents sound like they are being pointlessly difficult, much sympathy! This column by Captain Awkward about the derail that is "but I don't understaaaaand" might be cathartic/helpful. Link here:
https://captainawkward.com/2022/01/18/1 ... ou-mother/
I was in a similar position when I came out, my brother is about 4 years younger than me chronologically and at the time much more little-kid than I was even at the same age, maybe it's a little sibling thing? Regardless, similar dynamic.
What worked for me was taking him out for ice cream (I assume a similarly neutral location like the park or whatever would also work) to sit him down and explain extremely, maybe overkill-level clearly. I planned it out beforehand and wrote out what I wanted to cover.
Here's my talking points, as much as I can reconstruct them from many years ago:
1. I am a boy which means brother, son, etc not a girl, sister, daughter etc
2. I will tell/have told Mom, Dad, etc this so you don't need to keep it secret or anything. If they ask you about it, tell them to talk to me directly.
3. If you have questions please ask me, I will happily explain and I'd prefer to talk about it directly than you try to figure it out by asking Mom or similar.
4. I love you and this changes nothing about our relationship
In your case it might help to add explicitly that your parents might contradict this, but you are the final word on your own name/gender/identity so he can and should ignore them when they use your old name or she pronouns.