Farming, I too have been where you are now.
DD3 only said "Barbie
duck and no" (obviously the important words in life - Barbie before Mummy or Daddy
)when she started school! She had a statement prior to starting and went to a specialist unit. She could only say vowel sounds until she was about 7 years old.
I would not worry at all about the reading aspect, that will come when your DS is ready, you dont need to add any extras that are not important now.
We tried Makaton, but DD seemed to have her own version of acting out words to make us understand and appeared to have a "couldnt care less" attitude towards anyone else understanding her. 
It was tough going, but DD has always been a very happy girl and we have always said that her sunny personality would get her far in life. When DD started to be able to talk and to be understood it was as if she was making up for her 7+ years of silence! She just did not stop. Even now at 16 years old she has verbal diarrhoea frequently.
Can your son move his lips? Did he dribble a lot as a baby? Can he purse his lips as when you blow a kiss? If he cant move his lips too well it would be a good start to get him to start pulling faces and try to move his lips. DD was totally incapable of any mouth/lips movement. She loved the face pulling exercises, though! 
As for the future, don't worry. I worried that DD would never be a "normal" child/teenager/adult. Life is good for her now. Talking is not a problem. She does have dyslexia, which I think is probably linked in some way, so has struggled academically, but she is happy. None of her friends see any difference between her and them. Even now she can get some words muddled a bit, some sounds not quite right, but she laughs it off and everyone accepts her as she is.
A good relationship with the speech therapist is important and you may have to be a bit bullish to get what you need to for your DS, but fight for him and his future. If you don't, no-one else will!!
Good luck!! 