How about the Phillip Pullman "Dark materials" series? (I think they have both the unabridged books and in several cases radio plays).
If he's old enough/mature enough to start on the Terry Pratchett books there are several of the children's and young adult stories on there that are around the 10 hour mark I think (although most are short), and I believe the BBC radio play collection is still on there (which has several of the discworld stories plus "only you can save mankind" and "Strata", but they may be a couple of years too old for him*).
There is a full set of the BBC Narnia radio plays (much better value than the individual radio plays as the complete set is ~15 hours for one credit) if he might like them, I have memories of really enjoying the books at about his age.
It might be worth doing a search on the site with the results limited to children and young adult (to be honest the line there is often completely arbitrary**) and things like "BBC" or "play as over the years the BBC has done a bunch of radio drama's of some very good children's books and they're usually done to a high standard (as you'd expect as they were usually done for broadcast on R4).
On a side note one thing I hadn't registered before getting my audible free is how many offers they do on the books, and a lot of the time if you pick up the Kindle version (including in say a 99p deal) you can get the audible version for £3-4 pounds
It's something I've started to keep an eye open for as several times I've ended up picking up the kindle version and the audiobook version for less than the price of an additional credit because of that
It's one of the reasons I keep my audible sub open, as I tend to horde the credits until there is a 2 for 1 sale, or a book I really want in audio format, but in the meantime end up buying a book or two a month in their daily offers, plus tending to get the audio version when I buy the ebook one (although that doesn't require an ongoing sub as it's just added to your audible account/the offer shows up on the audible site when you find the corresponding title).
*I know I started reading Pratchett at about 12-13 with Colour of Magic, and didn't fully appreciate some of the secondary jokes until much later (but things like getting hit in the "'vulnerables" in Guards Guards got a chortle on first reading).
**I've seen both Authors and Librarians complain vocally about how publishers define what is "children's", "young adult" and "adult", often stating things like YA puts parents off looking at some titles for younger children whilst putting off teens/adults from looking at books that are good to read as adults, and even the publishers have at times admitted they often decide based on what they think rather than what the author intended or thinks.
