I'm a little confused as to how hearing him read will help her identify word finding and expressive language problems.
My middle son (also 8) has what sounds like similar issues. He is often hesitant to answer when spoken to and doesn't speak up in class. Yesterday he went up to his teacher after school to give her a letter he'd forgotten to hand in. He stood there for ages unnoticed and then when he did get her attention he just thrust the envelope forwards and said "letter". She is lovely and gently asked him to try and use a sentence instead of just one word to tell her what he wanted and he just looked at her blankly (which a common response for him).
I raised my concerns with last year's teacher, who was at great pains to tell me he is fine and they haven't noticed any problems at all at school. I was also told that I was making a fuss about nothing because ds2 isn't ds1 and doesn't have ASD. 
I know he's not ds1 and although he has some traits, does not have ASD and I didn't need a teacher who knows absolutely nothing about ASD to tell me that. Apparently because ds1 has SNs, ds2 can't possibly. [angry)
Oops sorry, went off on a bit of a rant there.
I have been monitoring ds's language and am pretty sure there is an issue there. As well as struggling to express himself, he seems to have a delay in processing when people speak to him and when I asked him whether he finds it easy to understand people he said he sometimes has to wait until the words get into the right order before he 'get's it'. He also still reverses some letters and lots of numbers.
We are currently dealing with another issue of his at the moment (hypermobility) and have been having lots of appointments about that, so I haven't pushed about the language thing as yet, but it's next on my list.
That said, my ds isn't falling behind at school. He struggled through reception and year one, was on the SEN register for additional group and 1:1 work with learning his letters and basic reading and writing, but by the end of year 2 had caught up and has been comfortably in the middle of the year ever since.
Check out your local PCT's website, because you can self refer to Speech and Language Therapy in some areas, although there will almost definitely be a long wait. Alternatively, contact your school nurse, HV or GP, who can all refer.
You can go private, but it isn't cheap. Any Speech and Language Therapist should be registered with ASLTIP.
Finally, you could think about asking your GP for a more general referral to check hearing, speech and anything else he/she feels might have an impact on his language skills, as there is often a lot of cross-over between different issues.