Has he/have you heard of pogies/bar mitts?
http://bikepacker.com/pogies/I've been riding so far with merino wool liner gloves under Swix skiing mittens -- they are split like a cycling glove would be except with the forefinger then three other fingers. I like this better for braking with my style of handlebars, and they were cheaper than cycling-specific brands :) The liner gloves are a big part of being warm enough. I'm a total freeze baby but have found this combo comfy down to 20 degrees F with wind (but I'm only riding 10mph).
I'm super lucky that my bike mechanic makes pogies, so she's currently crafting me a pair. So far everyone I've spoken to that has them absolutely LOVES them -- they're usually able to wear one less layer on their hands, have better dexterity, and don't have the sweaty hand problem that a lot of cyclists do once they're warmed up.
In general I've found snowmobile clothes and gear to be warmer and less expensive than cycling-specific gear. And constructed to deal with wind at higher speeds :) I'm in Minnesota so there are long-established venues for finding quality hunting and snowmobiling clothing, where most of the biking gear I've seen is in niche stores and quite a bit pricier.
Props to your son for biking through winter! I went to college in Moscow ID and biked year-round, but being Moscow my bicycling didn't involve anywhere as long a commute as he has :)