I don't understand the part about phone agents.

There's only one number to call for US and Canada. And being based in SFO, I guarantee you those agents can touch tickets departing the US.

Also, just because they're still selling seats doesn't mean the flights aren't full.

However, they should be able and willing to rebook him on any other airline once the delay hits 2 hours. The preference would be UA, but they can do any airline with flights.

https://www.aircanada.com/en/agents_...Operations.pdf makes it very clear they can do this.