Armchair psychologist here but my guess is that your ds is anxious about leaving you in case you aren't there when he gets home from school.
I wonder if this is why his days feel long and, when there's work to do, his mind wanders to you. Whereas when he is playing, he feels distracted so time passes faster.
I wonder if something as simple as a little locket with your photo in or a 'magic pebble' - something small that he can carry secreted away in a pocket could help. If you each have one, you could tell him that you will press your locket/pebble each time you think of him - and he can do the same with his.
And then, when he gets home you will see who pressed more! That will at least let you know how often his thought turn to you!
But you could say the pebble is magic, keeps everyone safe. Whatever you want. Just be super sure to let the teacher know it is not to be confiscated and he is not to be in trouble for having his hand in his pocket.
You might think of something better - a keyring on his bag, a wristband. Anything.
And - get him a watch so he knows when home time is. I would also let him know what time you do, say 4 things in your day.
So - at 11 oclock i will be having a coffee.
At 2pm i will load the dishwasher. Keep the same 4 things and times so when he is at school, he can look at his watch and know what you are doing.
You dont need to be doing it of course! But it might help him feel secure to picture you safe at home. And you have your magic pebble to keep you safe so he doesn't have to.