A couple of things. It’s too short to determine if it’s an American accent or not. Secondly there are tons of Ukrainians who immigrated to America, many of whose children or themselves have served so they may feel like they need to defend their Motherland or simply to stop Putin. Just like we have had Vets going to Syria or to the Kurds to fight ISIS.
If that's even the side he's fighting on. A Ukrainian military uniform and a Kalashnikov doesn't mean much in a civil war when Ukrainian military depots were raided and emptied by the rebels.