Altitude illness is caused by one thing and one thing along, going too high, too quickly. From sea level to 8400' is a very significant change in altitude even though 8400' isn't terribly high. It is certainly high enough to get sick, obviously.
The altitude at which you are sleeping is far more important than your highest altitude on any given day. People cruise 14K peaks like Rainier in day or overnight trips all of the time and typically don't get sick because they are moving and breathing hard the whole time, and their exposure is short enough they don't have time to get sick.
As has been said, each exposure to high altitude is different and past issues mean very little re: how you will acclimate the next time. Kids are particularly susceptible to altitude illness as their compensatory systems are not fully developed and/or the combined stress can be a lot more significant on a smaller, weaker body.
What diamox dose were you taking leading into the trip? I've had better luck with Diamox using at the time of need rather than prophylactic pre-loading.
When you feel AMS creeping in, going down is a good thing. A change in 3000' or more is typically needed to start to feel better once you are in the grip of AMS.
Next time, try and spend a night or two at a more moderate elevation, say 5500 to 6000 feet before you camp at 8K or above. Do some moderate or light hiking during those days but don't wear yourselves out. Active acclimatization is good.
Lots of other thoughts on the topic can be found in an article I put together for the Journal of Mountain Hunting last year.
https://journalofmountainhunting.co...ss-prevention-and-treatment-by-coley-gentzel/
Sorry the hunt got cut short and glad it sounds like your boy is back in good shape.