My son responds well to melatonin -we mix two liquid drops with a couple spoons of applesauce. He eats that as part of his bedtime ritual, prior to his nightly warm shower and teeth brushing. We were resistent to medications for his ADHD after we confirmed his diagnosis, but I'm glad we tried it b/c it
made a big difference for him.
I'd recommend that you try to educate yourself about meds so that at least by the time he starts school you are at least fully aware of the options. Do not deprive him of something that could be medically necessary out of some kind of misplaced fear or lack of information.
No amount of "willpower" or even consequences and rewards will make him behave differently if his brain is unable to make the connection between request and execution. His body is having uncontrollable impulses, his mind is constantly racing, and he is trying to cope with all this nearly every minute of his waking day. He may become incredibly frustrated and feel like he's stupid, which would be a terrible way to go through life. This will lead to all kinds of social and emotional issues later on. In any case, I'm sure you don't want him to be incapable of living up to his fullest potential.
Maybe start with the melatonin for now, but do consider the rest of what I said. For his sake and yours. Best wishes.