Whatever method you use, what you need is tons of practice and he will not want to do that unless it is fun.Thinks of lots of interesting ways to present words and be willing to give rewards - praise, stickers, sweeties, games time, outings, hobby items, whatever works.You want to get to the point where he thinks the chore of reading is worth his while.
One thing I used with my DC was a jar with folded pieces of paper in it with words on. Aim for a decent percentage of words they know with some they have to work harder on.
They pull out a piece of paper and unfold it, read it and trade the piece of paper for eg a sticker or chocolate button. If they haven't quite got the word, put it back in the jar after the session without them seeing so you can revisit it.
A refinement on this is to fill the jar with activities and then he can do the activity eg park, football, bike, swim, cook.
Never underestimate the power of success. I had a chart up on the wall where we wrote up words which the DC could read with a big tick and a sticker next to them.
Once the list started growing, it was a huge motivator, especially if my friends came round and I could say to them in front of DC (after priming them in advance) "Look X can read all these words" to which they were suitably impressed and asked for them to be read out and gave congratulations. A bit of self confidence does wonders.