I don't understand why he is doing this
OP, try thinking of things from a child's point of view. The world is a big and scary place. New things happen all the time. Adults are 2-3 times your height. Your emotions are huge and often overwhelming - you don't have a bank of decades' of experience to set them against, so a scraped knee is agony, a missing toy is a tragedy.
What makes this world feel safer is rules, boundaries and certainty. These are provided primarily by parents, but can also come from other carers and teachers. Having rules in place is like having a fence around your garden. Yes, it keeps you in, but it also keeps nasties out, and stops you from getting lost.
Kids actively want these fences (think back to your favourite teacher at school, I bet it wasn't the most lenient one). They need to know where they are. Kids will actively escalate their behaviour until they meet resistance, because that's how they find out what the rules are.
Once they've found a fence/rule, they don't take its existence for granted. They check, frequently, that it is still there and still strong. (Imagine your house is surrounded by wild forests containing bears and wolves, because that's how the world feels to children. You would check your fence - giving the panels a good kicking to make sure - pretty often.
What is happening right now in your house is this: your ds has grown up so far without much of a safety fence - they've all been easy to kick over, and they've been a long way out from the house. Now, suddenly, he's woken up and there's a great big safety fence quite close to the house. He is in "wtf?" mode. He's testing it, to see if it really is as stable and strong as it appears. There's never been one before, so expect the testing to be quite persistent. If it passes the test though, although he may whine about the fence spoiling the view, he will feel a lot safer, and be calmer and happier.
The process of fence- testing can look and sound quite unpleasant. Expect tantrums, wheedling, manipulation, insults, right through to possible hitting or throwing things. The verbal stuff - just ignore it. Do not engage with individual comments. They don't matter - the rule is the rule. Hitting etc needs immediate intervention, and keep it as calm as possible. Don't let him see you're bothered. Weather the storm of the testing. You have to be more stubborn than he is.
All this is to say that, although it's trying for you, your ds' behaviour is pretty normal. He's no problem child, and he'll settle down a lot when he feels like he can trust the new rules to be consistently enforced. Consistency is REALLY important - every time you stay strong and enforce the rule, no matter what tantrum/ manipulation he throws at you, you're teaching him to trust you.
Don't be scared to explain to the boys what is happening and why. You're setting them boundaries because you love them. They need boundaries to keep them safe and well and to learn how to be good people and live meaningful lives. They can thank you later.