I have a lot of thoughts around this topic, largely because I have put so much thought into teaching young people to shoot in my life, including the “highest criticality” of such consideration when doing so for my own son.
1) Personally, I cut my teeth on long action hunting rifles as a kid, but have grown to largely no longer have interest in the .30-06 I grew up, the first rifle I bought for myself - so it sits in the safe. My brother received our grandfather’s .270win which was the first rifle with which I hunted as a kid - again, it sits in his safe, unused. I hunted a lot of game with my .30-06, in over a dozen states, but now, I just don’t find use for it… I use a long action magnum when I go after big game out of state, and when I’m home in KS, I use a short action cartridge… so for me, a long action would be a mistake.
2) A long action also eats more powder and punishes more recoil than a short action round when you’re not killing something on the muzzle end. You mentioned converting to 284win if he doesn’t find hunting interesting - what’s he going to do with a 284win? That’s a pretty specific niche that 284win fills, which not many people pursue (and most being of older generations). Is he shooting LR BR or F-Class currently? So again, long action doesn’t tickle my fancy as a life-long, adaptable option…
3) In a short action, a model with interchangeable bolts/bolt heads & mags/mag box assy’s would allow access to WSM’s, 6.5 PRC, RSAUM’s for hunting as well as having access to 223/5.56 or some short action standard cartridge like 308, 7-08, 6 Dasher, etc…
4) Let’s be a LITTLE realistic about adaptability, just for a second. Beyond pleasure plinking and hunting, factory repeater actions don’t play most competitive games. Long gone are the days of sitting on a competitive firing line and telling a fellow shooter about your rifle which “started life” as grandpa’s hunting rifle. A guy MIGHT get away with that shooting PRS, but again, that won’t be a long action rifle, so I wouldn’t spend much time looking in that direction - it’s likely to be a pleasure plinker or hunting rifle. Not likely any other use - so make sure it’s good for pleasure shooting.
5) At 13, he’s not too far off of fully grown. He MIGHT be a couple inches shorter than he will be at 18, 28, or 58, and longer term under the effect of gravity seems to increase our burden on the scale, but he’s probably somewhere considerably north of 50% of his future body mass, more likely within 20-50lbs of his adult weight. Temporary adaptations to his rifle could make it grow with him as well as help him manage recoil today - and forever. Also long gone are the days where our only options were a youth model in 243win and a standard in 30-06 - and thankfully long gone with them are the days when it was acceptable (and seemed sensible) for a father to put a son behind a heavy recoiling rifle, saying, “suck it up, Mary, it doesn’t kick THAT hard.” A guy can stick a brake out front to reduce recoil, shorten the stock to fit shorter arms or buy an adaptable stock, use a smaller cartridge up front and rebarrel later - or buy multiple barrels up front - etc etc etc. But the easy button would be something in the order of 7-08, 243win, 260, 6.5 creed which will hang on to utility forever and will never have excessive recoil beyond its capabilities. I say often: deer hunting is best done with 100-140grn over 35-45grn powder at 2700-3000fps, and this isn’t age, gender, or physical size specific.
6) Of note - fathers give things to sons, fathers loan things to sons, and fathers leave things to sons when we pass. So if your son does find need for a more powerful rifle which you happen to have - especially a limited, one time need - he can borrow yours, or eventually, when you pass or get too old to make use yourself, he can have yours. This might be a gift, might be a burden - such as a dozen rifles and shotguns I have in my safe which were my wife’s or my fathers’ or grandfathers’ before us, and will be our son’s someday, for most of which we have to fabricate intentional uses, just to justify getting them out of the safe. Building what he’ll always use makes more sense than building something he might use, maybe, someday, if he likes a certain thing for a certain time…
7) Finally - what I did for my son:
At 2, I bought a Marlin 60 and a Savage Mark II 22LR, and I bought 3 spare stocks for each. I cut one set down to 8”, the other down to 11” which I installed when he turned 6, and the final pair to be left alone for when he’s grown. I also bought a new stock for my great-grandpa’s old 410 single shot shotgun, also cutting it down to 10.5” when he was 7, leaving the factory stock alone for when he grows. He started shooting AR’s at 5, naturally, with the carbine stocks telescoped all the way in. At 8, I ordered a Defiance Ruckus with two bolts and a Manners Compact Gen 2 stock. So I can put a 20” carbon fiber 6 Dasher barrel out front when it arrives (he’s shooting my barreled action out of his stock currently for this deer season), he shot my 6.8 SPC 18” and 10.5” the last three seasons. He’s asked to get a new stock for his grandpa’s 30-30 which will fit him, so we might have that in order. He shoots specialty pistols which don’t bring the burden of a stock to fit his body. He carries rifles a little heavier than he’d like so he can manage recoil better, and everything he shoots has brakes or cans. His Ruckus’ second bolt is a magnum bolt face, so it can become a 6.5 PRC if he wants to hunt with the rifle later.
(Same strategy I used for my own rifles - I have two boltheads for my Seekins Havak so I can shoot 6 Creed and 6.5 PRC. I also have a 458win mag barrel for my 300win mag, and my wife has 416 Ruger and a 338wm barrels for her 7rm).
For my niece, when she turned 5, I built two left hand Stag uppers, one a 16” 5.56 and the other an 18” 6.5 Grendel, so she could hunt with one and train with the other. Pretty good survivability on both of these options when mated with a carbine stocked lower.
Best laid plans of mice and men - manufacturing heirlooms isn’t a high likelihood game. But we can make sensible plans which have greater likelihood of surviving contact than less sensible plans.