Regarding the "Why now?" comment (and I have seen similar sentiments in other posts but this was the last so have used this one to quote so apologies @Beachhutnut as this isn't just aimed at you) the OP says in her post that her child's uncle has messaged on a few occasions despite the fact the OP had never replied.
This to me says the uncle has tried to open dialogue with the OP, has had no response but has kept trying despite that. This to me suggests that the uncle is doing his best to show he wants a relationship with his DN by continuing to try and make contact and showing he's willing to put in the effort whilst also trying to respect the OPs boundaries (IE when he doesn't receive a response he doesn't bombard the OP with more messages, I'm assuming at least or I'm sure the OP would have mentioned this).
As for the grandparents, we don't know why they haven't made contact but perhaps at first they didn't know how to make contact and then as it's gone on longer and longer they've felt like they shouldn't or couldn't make contact after so much time. Perhaps the uncle has told them that he's had no response to any of his contact attempts so they have decided not to put themselves through the emotional trauma of trying to contact their grandson and being stonewalled too. You also don't know what's been going on in their lives - there could have been some health issues or other serious family concerns that has taken up their attention and they felt that it wasn't the time to also be battling for the grandchild as they didn't have the time, energy or just weren't in the right place to be doing so.
For example when my DD was born I was living abroad but my parents were dealing with being carers for my maternal grandparents who had both been newly diagnosed with Alzheimer's/dementia. Other than them coming out to visit when DD was first born, I was the one making most of the effort with ongoing communication - sending photos/letters/video messages; travelling home to visit etc - because they were so utterly exhausted by caring for my GPs and advocating for them in the care system they just didn't have it in them to instigate the relationship, especially due to the distance, and we weren't estranged. Now, we're back in the UK and live 5 mins from my parents, my DD has the best relationship with them. They have so much more time and energy to put into their relationship with DD (and all my nieces and nephews) now that my grandparents have passed and my parents no longer spend all of their time and energy on caring for them.
Also we don't know how old the uncle's child is. They may be similar age to the OPs child and so at 6 they're starting to have playmates/friends etc and he's wanting to have the chance for the cousins to have that friendship. It might even be that the uncles child has another cousin from the other side of the family, the uncle has seen that relationship flourish and wants the chance for that with his child and OPs child too. Or perhaps the uncle's child is a baby and having a child of his own has been the catalyst to renew his attempts to meet his nephew because he wants his child to know his cousin and have that relationship (or for him to at least be able to say he made every effort to create that relationship).
It's really hard to be the "estranged" family as you are treading a very fine line and whatever you do will be wrong to some people - if you don't make contact then clearly you don't care and want nothing to do with the child, even if it's because you're worried about causing upset to the child; if you do make contact then it must be to help the absent parent get contact even of you have a LC relationship with the family member in question; if you make contact and then give up when you get no response then obviously you don't really care and there was something dodgy going on; if you make contact and keep trying despite no response then you're just harassing the resident parent etc - you can't win.
You have to consider what happens if your DS asks about his dad's family when he gets older - if he goes and looks them up and asks why they weren't interested, what happens if they tell him (truthfully) that his uncle tried to make contact but was ignored? That they tried to have a relationship but were prevented from doing so because you presumed they were all deadbeats like his dad? Even if you have done it to protect your son, knowing he could have had a relationship with his wider paternal family who wanted to have a relationship and was prevented from doing so by your actions may cause a whole host of issues further down the line.
OP, personally I think that if your Ex's brother has made sustained good faith attempts to make contact with your child via messages, despite getting no response, then the least you can do is meet him (without the children) and have a discussion about how you want the relationship to progress or not.
That might be that you and the uncle agree to a slow introduction - he can send letters/cards/gifts for birthday/Easter/Christmas and you'll talk to your DS about who is sending them, then maybe progress to meeting in the park/cafe at regular intervals, then having the children meet up and getting to know each other. Equally, you might not be ready for your DS to meet him so you just agree to open communication with the uncle - so you agree to message periodically with updates about DS eg any achievements/photos of him in the school play etc and the uncle can respond to that and let you know of any life updates of his own, ask questions etc with a view to progressing to eventually meeting up. Even if you end up deciding you don't want anything to do with your Ex's family, at least then they can get closure (whether that's a "never gonna happen" or a "I'll leave it to DD to decide when he's older") and know where they stand.