I can relate. I retired and found my inner Cowboy. Cowboy action shooting became my passion. About a year ago i was diagnosed with cataracts in both eyes, but my right eye is really bad. It was either quit shooting or learn how to be a lefty. A bit of a tough learn, but I persevered and I'm still in the game.
I've been an 870 man for a very long time, it will work left. As will an 1897 Winchester. But the one that surprised me is the Model 12, the slide rlease is perfect for a lefty. The safety is wrong, but the rest of it works fine. Browning makes a pump that ejects out of the bottom like a Model 37 Ithaca. It's also available in all the gauges. It is made to handle steel with screw in chokes.
Try a few in the gunshops. of all the ones mentioned, only the BPS is current and available. The 870 may be in production again, but I've not seen one. A Model 12 is affordable used, but steel and finding a slug barrel may be an issue.
When my son, a RH shooter was young he started with a 410, then a 20 and 12 soon after. I loaded 12 ga lite for him, but they grow up fast, don't even remember the load anymore. You should have a good recoil pad, and the gun needs to fit, length of pull. Back in the day Remington sold youth stocks, I bought him a set. The good thing after he outgrew it, selling the gun at a show was so easy. It went to a youngster that smiled like a Cheshire cat as he and his dad walked out.
The big thing is how the safety works, Mossberg has a tang safety, which is good, the rest of the gun is poorly assembled IMHO. My son's wouldn't even shoot out of the box. Now that is a real disappointment for a young boy. Dad to the rescue, I filed and hammered until it worked, the pressure was on.
As one that has switched from right to left, you may want to have him learn to shoot right, it makes buying a good gun so much easier. My son had left handed tendencies as a boy, I discouraged that. The alphabet/rainbow people may be upset with me, but with a safe full of RH guns and only one son, I didn't want that foolishness if it could be helped. Being able to shoot left or right equally well has it's merits.
A Colt SAA has the trigger offset to one side, try shooting that LH, it sucks.
One last point, the cast on some shotguns will make them hard to shoot left. Most pumps are neutral, shoot equally awkward LH or RH. The 870 has a bit of leeway in that respect, you can get about 1/2 inch cast on/off with a bit of work on the stock. I've also increased the drop to fit a nephew. Last is the pitch, for chickens and cowboy work, you want at least 5º. For high flying ducks and geese, perhaps less. Women and hefty men can use even more pitch to prevent getting pinched in the sensitive areas.
https://theyorkshiregent.com/shooting/gun-fitting-guide-fit-shotgun/