Glad to hear it's not just Americans who are careless about COVID.

I would sit SS down next time he's over, and give an "Explain Like I'm 5" description of the immune system and why it's important for people to be careful with illness around babies in the first few months of their life. Tell him that you love that he's such a good big brother and you can't wait for the day when they can play together.  But for now, you need his help in other ways that don't include touching the baby, especially if he himself is sick.

Leave BM out of it, explain a little about COVID, and then YOU enforce it. Stop relying on your DH to do it. (You said before that SS likes you, so he might listen. And if not, you protect your baby.)

Hey, if he doesn't want to wear a mask he only has to get tested.. 

I remember when my DS was a newborn. I coccooned in a bedroom with him for probably the first three or four months of his life. And there was no COVID happening then! Being alone for the most part with a newborn while my DH worked  was painful in some ways, but it was blissful too. I'm sharing this memory to point out that I think your feelings are justified and reasonable.Protect that baby! 

If you are able, socially distance on the weekends your SS comes to visit. I mean socially distance with everyone in the home, including your DH. Tuck you and baby away in a bedroom. Avoid sharing air. Sit 6 ft apart at meals. Wear your mask in mutual spaces. .You can do the explaining to SS about germs and what not, but I also think that before he you even allow him into the room to visit the baby, he must wear a mask and he must wash his hands. 

I often washed hands with SD before I allowed her to be near baby DS, because she often lied about washing.

I agree it is wise that if SS is symptomatic, he must have a negative test before visiting.

DH should wear a mask during visits. I do not think you'll have much luck getting BM to test SS eow. 

Update? How did things go?

I only say this to help ease your sense of panic, I've been on the phone constantly with my child's pediatricians because my own SS's BM had covid and still confined to forgo of making him wear masks and reminding him to have good hygiene and be aware of germs that are constantly passing. The pediatrician told me the virus isn't really being spread by younger children. If my bio gets covid it is more likely to have coke from a grown up who's spreading it. Now take that for what you will, not all drs give them same information but if it helps your peace of mind a little bit I hope it helps. I know some people are saying don't blame bm but 9 year old boys (my SS too) are gross. They need to be reminded to shower and brush their teeth regularly so now in a pandemic it's a parents job to be overly assertive with handwashing and mask because that age just won't do it on their own. The part of their brain cannot comprehend the seriousness of consequences because that part of their brain hasn't developed yet. 
 

all that being said, I don't think you should be overly paranoid and never let your SS touch his sibling. That's alienating and going to cause a whole bunch of issues. I personally keep hand sanitizer in the entrance way of the house for when we all get home we can sanitize. I also keep some in the car when I'm picking him up from somewhere. As long as you're making sure he's washing his hands everytime he sneezes it will be ok for him to be around the baby.