It sounds to me like conjunctivis could be developing. That is hard to see at that stage for the observer, maybe a little red and nothing more, but can feel like grit in the eye. And it DOES improve during the day, to be worse next morning again. You can get it in just one eye but it can easily be spread to the other eye. It is highly contagious.
If it is conjunctivitis it will soon be crusty in the mornings and if it gets worse it can glue his eyelids shut (warm wet facewasher will help, but don't let anyone else touch the washer afterwards - microwave the wet washer in a plastic bag then let it cool, to kill any bugs). Bathing with saline is also good - you can warm it if you like, either by holding a plastic ampoule of it in your hand or sitting the ampoule in a cup of hot water. Using an eye bath - could be a bit confronting for difficult child, but if you can persuade him to, then saline at body heat will be the least uncomfortable - more comfortable than water. The saline to use - sterile, should be available in plastic ampoules from a pharmacy, labelled "0.9% NaCl", "0.9% saline" or "0.9% sodium chloride". It's also about 0.15 Molar, if they use molarities instead of percentages. You half-fill a small eye-bath, lean over and hold the eye bath to your closed eye, straighten up and look up (with the good eye, the other one has an eye bath held to it!) then open the eye which should now be covered in warm saline. Blink a few times to get the saline all over the eye, to wash it. Then tip forward to remove the eye bath without it all going everywhere.
I remember getting conjunctivits when I was a kid. The first attack really scared me because I couldn't see - my eyes were glued shut and I had no idea what time of day it was. I remember screaming in panic, until my mother explained what it was and helped me bathe my eyes. Next morning they were completely fused shut and I had to feel my way to the bathroom and grope to find a washer. I finally washed enough gunk off to be able to force my eyes open (removing a lot of eyelashes in the process) to find it was only 3 am and too early to get up. I went back to bed again and when woken at 6 by my mother, my eyes had again fused shut.
Since then I've had this enough times to know that what I remember, was a really bad dose of it. When it comes on slowly you often don't get to the crusty granular eyes.
If the eyes aren't crusty they still can get weepy (clear rather than opaque) and this may be viral rather than bacterial.
difficult child 1 is on the tail end of treating conjunctivitis with antibiotics - he developed a dose of it a week ago.
Or talk to a pharmacist, if they can help. It depends on what you're allowed to buy over the counter. Here, we have to see a doctor to get the antibiotic ointment.
Red conjunctiva can also be from rubbing eyes or crying a lot. But to be sore like that - it could be a scratch, too.
Marg