I am 43, right handed but left eye dominant. I started shooting LH from the beginning, maybe 8-10 yrs old. It just felt natural, and there was nothing to unlearn. If you start him out lefty, it is zero adjustment, IMO.
I grew up bird hunting and shooting squirrels and things around the neighborhood with a pellet gun. My father bought me a Browning BPS which is a bottom eject shotgun, so it is perfect for lefties.
When I got into big game hunting in high school, I shot a RH rifle but shot it lefty, and honestly there didn't seem to be any difference. I didn't know any better, and it is not like I was trying to cycle the bolt as fast as I could for a follow up shot like I do with a pump shotgun while bird hunting.
I used a RH rifle for years, before I knew anything about cast-off, cast-on, etc. I now shoot lefty rifles to ensure the stocks are in my favor - I think if you get a symmetrical stock you will be fine with a right handed rifle. Just don't make him fight the geometry of the stock. If you get a LH stock even better.
If you start him out lefty, he will become used to it and, IMO, it is the best thing for him. Don't make him try to fight the stock or his eyes. I think you are doing the right thing by considering this now and can get him started the right way from day one,