Sympathy for the 8-year old. Of course. Wish life were easier for OP. Yes, but the crack about "UA made the kid cry" is just blame-shifting. The rules are clear. And, while having the passport checked at another station made sense based on these particular facts, it screams out international child abduction / passport fraud (not an accusation against OP, simply pointing out reality) to check documents without the 8-year old physically present.

OP should immediately make certain with UA that his son's onward IAD-LHR-XXX-ANC is still in place. It may well have been cancelled at ANC when he did not board. It can be reinstated, but sooner the better as the flight could be overbooked.

On return and not before, I would put together just the facts without recriminations and accepting responsibility for the error, e.g., no passport at ANC, ask UA to reprice the son's itinerary IAD-LHR-XXX-ANC and see if it comes out less than what OP paid. I would also ask UA to consider refunding or providing a credit for the UM fee since it wasn't used.

UA has no obligation to do so here, so all of this is about tone.