Thanks for the excellent advice!
My initial thought was that I would ease into backcountry / sidecountry, not really long alpine tours, mostly extending the resort season and see where it goes from there. 2500ft per day sounds about right - for now. From what I've priced out, I can get mentioned bindings (frame type), skins and mount 'relatively inexpensively' for about $400 per setup on my existing skis (same for my sons) during off-season sales. Going to a true "AT" setup, would certainly cost many time that amount. I personally don't have any real goals regarding backcountry / sidecountry, and will probably still spend the majority of the 'season' at the resort. My son, I'm sure, has loftier goals. Every time I've gotten into a new sport / hobby, I have followed a similar thought pattern (skiing, Mountain Biking, Hockey) I 'eased' into it with 'lower end' gear, than got hooked and ended up pulling the trigger on much better performance gear - so chances are, you're right, I'll just end up buying the 'right' gear in a short amount of time. I'll definitely give it more thought and any more advice is certainly welcome.
And yes, we do have beacon, probe, shovel, practiced a lot in and around the resort this season with friends that were more experienced. Planning on getting AIARE cert next season. Staying on low angle terrain for now - specifically to stay away from higher risk / higher consequence terrain.
Question about frame binding 'compromise': I can definitely see the compromise on the uphill, but what about on the downhill ? Any slop in these mentioned bindings, or any chance the heel may inadvertantly release (from ski - not boot, inadvertantly going into 'touring' mode) - my son does ski some more aggressive lines, hitting drops ~ 15ft, etc - wouldn't want any compromise there, especially as he advances further. I demoed skis with Marker Barons at the resort a few years ago, and didn't really notice any difference in downhill performance from my Marker Griffons - but that was just 1 day at the resort, was really more interested in the ski at that time and they just happened to have the Baron bindings.