Yeah, I think I'd find another chicken related job for your son rather than confine them with hours of sunlight left.
Maybe before bed, he could fill water and feed like others have suggested. Maybe he could turn the litter over (if you use the deep litter method) or some other maintenance thing. I'd let them naturally go to bed with sunset, though. For one, it's just easier! But most importantly, it'd reduce their stress rather than being cooped for a couple of hours before nightfall. My chickens are usually awake for a little while before I get out to let them out, but I think they'd be stressed and mad if I tried to confine them that long before they were settling in for the night. It's also a LOT easier when they put themselves to bed at sunset and all I have to do is close the pop door rather than trying to wrangle them all inside before they're ready.