Hi, I have a son a little like yours. Some kids need lots of physical activity to stay focased and eventually sit still. Their little engines are running on high and they need to do something to slow it down. After school (and before it becomes a problem there) have some activities for him to do. (sitting at school and then sitting playing with the computer and then sitting at dinner and remaining calm . . . wouldn't happen for my kid either). Get him running, jumping, skipping, rolling (which is vertical spinning), climbing, etc. Put him in swim lessons or start him in indoor soccer. Or just run races around the house. Set up an obstacle course, etc. A little effort to get him going in that direction will pay off big time. When it is warmer (maybe it IS already warm where you are at)--- hit a park for 45 minutes or a half hour (or longer if you can)---- let him run, climb and swing (very calming). All of these feed the nervous system and calm it back down.
My son has sensory issues and texture and chewing are two things that affect him. We just started the "trying bowl". If he (or my other son that eats everything--- but we have to be fair and not single out my sensory kid) finds it by his plate, he has to try what is in it. I only put one bite in it of one thing. He doesn't have to feel like I am going to make him eat a whole plate of it. So he can try it for texture and taste stressfree. I make it like he is an adventurer to try it.
Good luck and let me know if you need any other ideas for an overactive engine.
Physical activity has a direct effect on behavior.
Food makes a big difference in my kids behaviour. Mainly Omega 3 (Udo's Oil with DHA) in a smoothie every morning and a little protein every few hours, even just a couple of small bites of meat. They argue less, are calmer, cope better, and more cheerful when I do this.
The timing of your son's trouble seems like he needs protein. It's best if you can give it before they start feeling that way, and prevent it, that way they don't need to recover.
Hope these small ideas help you too, they are pretty easy!