So, for anyone else looking at this thread for advice, I went for the Felt Q24 in the end
And it seems pretty spot on size-wise - he's obviously towards the small end of the sizing but he can ride it fine. The bike is nice - proper mtb parts on it. It's not as light as an Isla bike but it's not overly heavy. The fork seems about right in terms of spring rate.
The only negative I can really come up with is that the wheels needed some truing though it's worth noting that I got the bike in a box straight from the distributor so it hadn't been PDI'd or anything beforehand.