"He can read well but isn't very fluent and will often add in or miss out small words."
My DD went through a phase of occasionally missing or adding small words, and sometimes guessing at bigger words. With her it was for a different reason, as she was trying to maintain the flow and the expression whilst reading - she loved to 'tell the story'. Having read lots of posts in the Primary Education board on Mumsnet, I understood it was important to be totally accurate, even if that meant slowing down a bit, so any time my DD slipped up, I'd ask her to reread that sentence again. She gradually grew out of the habit of making the little mistakes.
One thing she did for a while was pause for a while at the top of each new page, and I realised she was reading it through silently. She would then read it out loud accurately, with appropriate inflection. Perhaps you could encourage your DS either to read a short passage silently in his head to prepare himself before reading out loud, or instead to have two tries at reading it out loud, where the first time is focussing on the accuracy of the reading, and the second time should include more shape and characterisation. That might also help his understanding of what he's reading.
"If we read a bit and then ask questions he would shrug.. or say I'm not sure.
If we push him a little he will get cross and say he can't do it"
Perhaps you could make the questions easier, eg "Is the character feeling happy or sad?" so he has just two options, rather than having to describe it himself.
Also, perhaps see if you can encourage the humour. Use silly voices for different characters whilst modelling reading to him (for example with you and he taking it in turns to read a page), and encourage him to do the same.
What about 'choose your own adventure' books, where your DS can influence what happens next?