For a list of reasons that are too long to go into here, safe to say that I’m not a fan of single shot rifles for youth. Too fiddly, too fussy, and the ones with exposed hammers introduce a whole new set of issues that is just not needed for young hunters.
Also, the single shots are even less desirable now that we have so many entry level rifles on the market that shoot great and are much more functional for the money invested.
Given your needs, I think it would be very difficult to beat a Savage Axis Youth Compact in 243 AND the cherry on the top is they make it in a left handed version.
You can even get the same setup in the Axis 2 model that comes with an adjustable trigger. The utility of this upgrade, IMO, would be questionable with a youth shooter. Personally, I’d lean toward safety rather than trying to go for the accuracy enhancements at this point.
The really terrific thing about the Axis is aftermarket parts are readily available, so a few years down the line when your son is bigger, you could ditch the plastic youth stock and go with a Boyds laminate stock and ta-da, his “youth” rifle is now an adult rifle.
Ruger makes left handed versions of their American, but none in youth or compact dimensions.
I know cartridge selection is a real can of worms for youth rifles. Let me just say that I own a .243, shoot a .243, and know it to be more than capable of bringing down the biggest whitetails MN has to offer. My father stoned a big bodied 6 point a few years ago and he was stunned. He was using my 243 after dropping his 30-06 as he had no time to check the zero on his rifle. He knocked the deer over sideways and it never took a forward step after being hit with the 100 grain Hornady SP.
Anyone who says the .243 is not a capable deer round is off their rocker and has probably never even shot one. I repeat my long-held view that if the .243 Winchester was called the 6 MM Winchester Magnum there wouldn’t be the slightest objection to using it for deer in these Magnumitius days.
Grouse