Let me point you to Sword Art Online Progressive

Here is the mobile version of your link

The original SAO LN in Japan is way past the anime. (A good 2 seasons or so.) In English, it's been published up to the end of the first season with the first arc of the second season coming out on the 15th. (Phantom Bullet) The anime adapts through to volume 8.

The SAO: Progressive series is a re-write of the first arc and hasn't been adapted beyond a single chapter. It's a really good read for those that are interested in what SAO should originally have been. The manga version of it is a silly adaption where it tells the story from another character's perspective while focusing on comedy.

Not sure about other kinds of LNs he might like considering it's hard to get an accurate sense of taste just from the series listed. Going based off the genres, Danmachi is a really enjoyable growth adventure action series I consistently look forward to the next volume of.

A Certain Magical Index is a series I love as the story is always becoming more and more exciting with some of the best writing from an author out of many light novels. The English publication isn't past the anime yet but the anime does butcher some of the arcs so it's worth it to start from the beginning. A companion to this series would be the A Certain Scientific Railgun manga which is also very action orientated.

Tate no Yuusha is an LN series without an anime adaption that got licensed in English. It's a pretty good adventure series that he might like. It's a safe bet for something he's probably not experienced yet I think. Only 2 volumes published in English so far.

It's not available digitally (yet), but you can buy it off of Amazon or some book stores.

Also don't forget SAO Progressive. https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/0316259365

Covers a metric fuck ton of stuff not seen in the anime or the main series of novels. It's literally Kirito and Asuna going floor by floor through Aincrad. In the English translation we just finished Floor 4!

It's released April 21, 2015 in English.

I was talking about the SAO Progressive novels, not the regular ones

Volume 1 and 2 of Sword Art Online are available at Amazon (and other retailers). Volume 3 and 4 are up for pre-order.

Sword Art Online Progressive, Volume 1 and 2 are both up for pre-order as well. (Volume 3 isn't even out yet in Japan.)

You can get the progressive novels on amazon for around $14. Currently volumes 1 through 3 are in English which goes up to the 4th floor of Aincrad. There is also a manga that follows Asuna instead of Kirito.

Here, I'll help you add to your collection!

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SAO Progressive vol. 1 (Light Novel)

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/0316259365/ref=sr_1_3?ie=UTF8&qid=1542854838&sr=8-3&keywords=sword+art+online+progressive

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SAO Progressive vol. 2 (Light Novel)

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/0316342173/ref=sr_1_5?ie=UTF8&qid=1542854838&sr=8-5&keywords=sword+art+online+progressive

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SAO Progressive vol. 3 (Light Novel)

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/031634883X/ref=sr_1_7?ie=UTF8&qid=1542854838&sr=8-7&keywords=sword+art+online+progressive

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SAO Progressive vol. 4 (Light Novel)

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/0316545422/ref=sr_1_1?ie=UTF8&qid=1542854929&sr=8-1&keywords=sword+art+online+progressive+4

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SAO Progressive vol. 5 (Light Novel) - JUST RELEASED

https://www.amazon.com/Sword-Online-Progressive-light-novel/dp/1975328140/ref=sr_1_1?ie=UTF8&qid=1542854838&sr=8-1&keywords=sword+art+online+progressive

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SAO Progressive vol. 6 (Light Novel) - PREORDER

https://www.amazon.com/Sword-Online-Progressive-light-novel/dp/1975383338/ref=sr_1_7?ie=UTF8&qid=1542854935&sr=8-7&keywords=sword+art+online+progressive+5

Yep! Here it is.