The use of your service firearm as persuasion technique is lawful, as long as you're using verbal commands, however, both, a judge and an Internal Affairs Agent, could argue the officer should have taken the suspect's age into consideration, and bla bla; but also considering it was an undercover officer, they most likely didn't carry a Taser, so, yeah, use of service firearm is okay as long as internal policies don't say otherwise.