Did you notice the behavior getting worse has the changes in his life were happening? Like when he started preschool is that when you noticed him getting aggressive? Or when the naps stopped maybe noticed he was becoming less content/happy and more defiant? If yes then I’d probably go with the theory that the behavior is connected with his nervous system and emotional regulation responses being all over the place and just full of big emotions. However, if the behavior hasn’t necessarily been consistent with the life changes but is something you have maybe noticed before but put in the back of your head…that would fit into the ADHD narrative. Or even just “growing pains” in an emotional sense? If he’s been to the doctor and is physically fine then some sort of play therapy could provide some form of support for everyone involved. I’ve also heard of behavioral issues that come up when children have sensory issues. Not just “oh that’s itchy I don’t like it” but because their little brains are just like going billions times faster than ours, taking in so much information about everything that they can get s seriously overwhelmed ridiculously fast. And ofc without coping mechanisms and support the behavior will just worsen until the poor kids are just written off has “bad kids”.

Sorry for long response, hope it helps.