PECS was brilliant for my DS, but we only started it when he was 3. Earlier would have been better. He took to it really well and the SALT at his special preschool filmed him for training purposes. (Proud!)
It worked for him because he found language and signing too abstract and also too fleeting. Using the cards with photos or symbols on was much more concrete. He would ask for a favourite toy or biscuit etc by passing you a card with its picture on, initially. Got him to realise that communication was useful. Before that he just didn't see the point of asking for something. If he didn't get what he wanted he'd just get upset. It might have been that he just thought I knew what he wanted and couldn't understand why I wasn't complying. Once he got into the swing of passing a card to me and getting the item, he realised that he needed to communicate his wants and desires, if you see what I mean.
I would speak his words for him. I'd offer him a choice of maybe 2 things with their picture (and words) on 2 cards. Maybe a crisp and a small piece of biscuit. Hand over hand I'd get him to pass me or my OH the card with a biscuit on and I'd say, 'I want biscuit.' Once I had the card from him I'd give him a small piece of biscuit. Same again with crisps. Once he got the hang of it he could choose the biscuit or crisp card and pass it too me, me saying 'I want crisp' etc.
I made lots of cards of pictures of his favourite toys next and gave him choices. It really was a turning point for his communication.
That's just a very simple idea to try initially. The whole PECS system is in more depth and has proper rules to follow but hopefully you'll get an idea of how it might help.
My DS2 is 18 now and speaks well, but still chooses not to interact much, but he's happy and doing well. 