Actually, believe it or not, English is not regulated by a governmental or pseudo-governmental organization and thus is one of the many languages where the only authorities are descriptive and not prescriptive. Basically meaning that now, according to almost everyone, "Someone lost their backpack" is now grammatically correct and "Someone lost his backpack." is potentially incorrect.
1) it would be great if English had a standalone neuter singular third person pronoun, but English sucks.
2) It's probably better to say "Someone lost this/that/a backpack" instead of using a pronoun because, since someone lost it, ownership is implied, and we don't have to go through this shit ever again.