A couple of thoughts... The real question is what is your son going to use that rifle to do - poke holes in targets, or harvest live game? Once that question is definitively answered, the rest will follow.
As an example... For my youngsters I loaded a .50 Hawken with 20gr FFg Goex under a .490 patched round ball. It was a hoot to shoot and clean enough that we could easy go 30 or 40 shots without significant fouling buildup.
Same story for my Traditions inline .50. I don't shoot sabots but rather shoot 285 grain minnies through it. The kids found it very shootable at 25 grains and quite accurate at 50 yards.
NONE of the above loads are sane or recommended for harvesting live game. For that, the in-line gets 90gr FFg and the Hawken gets 70gr FFFg - in both rifles these loads represent the crossover point between down-range killing energy and accuracy/shootability.
I have only shot sabots a few times. Around here our shots on deer are short - the Hawken is subject to a self-imposed range limit of 50 yards on whitetails while the in-line will stretch to 75 yards if needed (but it's never been needed). So far my longest shot with a m/l rifle on deer has been 50 yards when an impudent little spike buck literally stood on top of my 50 yard range marker. Needless to say I had no excuses for a missed shot since I knew with exactitude the range to my target!
I would suggest you consider shooting non-sabot loads to ease the loading process and reduce the need for a whistle-clean bore. Doing so will likely result in more shooting being done with greater pleasure and lower cost.
Oh, I cast my own .490 round ball and Minnies using a Lee mould and pure lead reclaimed from spent .177 airgun pellets. Works a charm and the deer never seem to argue with it!
If you are starting a recoil-shy son, I would clearly step away from sabots entirely and, using minnies, drop the powder charge as low as you can go. In the above posts you've seen mention of 25-30 grains of powder - this is a good place to start. As a child I started shooting waaaay too much gun waaay to early and have been battling the resulting flinch reflex my entire life. It's no fun. Start 'em early, but start 'em light.