You have recognised the issue and are rightly looking for strategies to deal with it. It’s hard without knowing anything about your autistic son.
Your younger son deserves a lovely birthday where he feels special. He may grow up resenting his brother. Years of “ let him have it” “ let him win” etc.
It’s not good preparation for life or building resilience.
I taught a child with very complex needs and autism in special school. His siblings attended the mainstream next door and Mum would collect him with them in tow ( most were on transport home)
So we witnessed cakes made by siblings proudly shown to Mum snatched and eaten, toys taken and pictures destroyed.
Mum did nothing and we understood why as our pupil would have had a meltdown and she couldn’t have got him home.
We saw this but I know it will have gone on in lots of homes.
Strategies will depend on his understanding and ability. If they have 2 toys the same would he still want both?
Social stories may help as PP says
Turn taking, sharing and waiting are best taught with an adult first using timers and a WAIT symbol
Name ownership of items verbally and visually smaller items can be put in zippy wallets named e.g.Ben or with a colour if he can’t read
When playing alongside use coloured hula hoops 2 different colours everything inside the red is Ben’s blue yours etc just to teach.
Colour coded toy boxes again while learning to teach ownership and possession, one colour each.
Use books about turn taking, sharing, being kind etc. Even if he learns some skills in school he won’t necessarily transfer them to home.
Have somewhere secure for your young DS for toys. Do they have their own rooms?
For his birthday could the birthday boy have a trip out somewhere with you or Grandparents for some special time away.
Can he open presents away from his brother ( not forever) just til things improve.
Things should get better over time especially as you are looking to address it. Good luck.