Name-changed as potentially quite outing.
We had a child at DD's nursery who was periodically very aggressive and defiant. It sounds a bit different to your situation in some ways but not vastly different in others. The nursery tried all sorts of things to stop him hurting other children and one quite helpful thing introduced for all the children was "Tucker Turtle" about what to do when you felt angry, and focused on lots of books about different emotions and how to recognise and deal with them. It was slow-going but I actually think that such an intense focus on emotions was incredibly helpful for my (very rarely aggressive) DD too - the vocabulary and self-control it gave her is now beyond what I expected for a child her age.
The boy is now at school now and doing far, far better, I understand. How much is down to what was done and how much to just growing out of it, who knows.
I am actually of the (very amateur) opinion that he's a particularly sensitive boy in need of a ridiculously over the top amount of affection and positive reinforcement alongside firm but explicitly loving, and 'unbothered' correction (if that makes sense). Essentially he needs key caregivers to have absolute, unquestioning confidence that this is in the range of normal (which, by the way and from what you've described, it is), that he's a lovely boy really and he will - with their love and guidance - blossom into that lovely boy with kindness and consideration for others and to treat him accordingly.
So much easier said than done as a parent with other focuses and with so much skin on the game. It would almost be easier for a very experienced, old-fashioned nanny who's totally sanguine about it and her confidence in him, and in herself to deal with anything he throws at her.
If you're able to channel (fake!) that sort if approach, though, I think it helps to lower the temperature and feeling of being out of control that might be internally scaring your child, particularly if it seems that his caregivers also have no idea what to do with him, or are stressed by his behaviour.
As you and others have said, active praising of any good behaviour is helpful, as is an over-abundance of affection, over-preparing verbally about concrete things he's going to do to behave well before going into a situation, and polite bafflement when he acts out because you know that "he's such a kind / thoughtful boy and kind / thoughtful boys do X, Y and Z" etc. but we'll try again in a bit once he's calmed down type thing.
I hope that some of what I'm saying makes sense. In my experience it's a long process but it's basically a project in managing your own emotions as much a his and faking (if necessary) an absolute calm confidence and loving approach that will get you close to where you want to be. For that you'll need some support yourself.
It's hard, but you're clearly a loving, concerned mother and I'm sure it will all come right. Good luck!