"Él se fue con su mochila al hombro."

Good question. "Ir" has the specific meaning of going somewhere, whereas "irse" generally denotes leaving a place. "Él se fue" has the meaning of "he left" while "Él fue" has the meaning of "he went." I hope this helps. A subtle difference, albeit an important one!

Duo accepts this translation now.

And I put 'he went with his rucksack on his shoulder' and just waited to lose my last heart, as previously it wouldn't accept rucksack - and yay! Accepted!

Accepted 5/18/2014

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Good question. Anybody?

You could try reporting it.

Mochila all by itself can mean backpack.

Also, "espalda" is "back" though, interestingly, I inadvertantly found that (and I am not sure how correct Google Translate is) "shoulderS" plural is translated as "espalda". Maybe because if you have something on both shoulders, it is really on your back?

backpack (or rucksack) on his shoulder would be fine.

Él se fue = he left but él fue = he went, so that's probably the biggest issue.

Mochila usually means backpack (or rucksack) in my experience. Apparently, it can also translate to shoulder bag or book bag in the UK.

Went out = salir