Reading a wider range will come with time - my ds (end of year 9) now reads a far greater assortment than he did before. Try mixing in some non-fic if he likes science, I bought ds one on things science can't answer and he's glued to it. Eion Colfer has just written a Hitchhikers Guide thing a la Douglas Adams, and that got read in a couple of nights. I tried with the books to approach it in a different way...ds has a subscription to Audible for Christmas each year and he can download two books a month, sticks them on his ipod and listens. Listenig is one of the key skills I think in English, so if nothing else, you're helping him there.
Be thankful that he does read!
It sounds like he has Year 8 dip, which is a bit of a boy thing and sent me into panic last year. A lot of it is to do with maturity, my ds seems to have grown out of it and now tries and expands his answers, as I have pointed out that nothing to mark means few marks at GCSE and crap sets lower down the school. (I'm a senior examiner for one of the exam boards so know whereof I speak, and he does listen when I tell him things like that.)
If he's a bright boy, then he may be getting all the geek and nerd name calling...the answer to this is that geek shall inherit the earth and make a bloody fortune on the way. I keep telling mine that brains are something you always have; being popular at school is fine til you leave.
The conclusion I have come to at the end of Year 9 (and I have been a form tutor), is that boys do a lot of 'hidden' growing up during this period. They have their comfort zones, but can be coaxed to try different things. The ds I have at the end of Year 9 is the one I thought I'd lost during Year 8, so they do come back.
A book that might help are Blame it on my Brain; readable for both worried Mums and confuzzled year 8s who know somethig is happening, but they aren't sure what! I find lots of toast, hugs and generally being there helps as well.