Even if you don't want to try PECS, you could try little sentence strips for the two word combinations so he gets the idea? e.g. eat apple, drink juice, red car
I worked with a little boy who enjoyed the rhyme "bobbing up and down on my big red tractor" - we had pictures of different coloured tractors to begin with and a two stage symbol strip so we would pause and say
bobbing up and down on my big...
and he would choose a colour and put it on the sentence strip e.g. red
red tractor etc
After modelling/pausing/making it a communication temptation he would say red tractor and then we changed the next verse to chugging up and down on my big green train etc.
The faster/more frequent you do whatever it is, as Star said, the better.
You also have to find something that's motivating for your LO, that's the number one key to success - if it's not something really wants he's not going to have that motivation to add the extra word in.
If there's something he requests a lot, you can try adding a please onto the words he uses to request, either a word or a sign e.g. choo choo please. When he requests, model the two word phrase and wait. A sign can be a useful prompt here, with another adult acting as a model to show that the two word phrase "gets" the prize.
Again, signs are a bit hit and miss kids who may be on the spectrum. Some kids fly with them and I think they have many advantages but not all do, you just have to try it and see.
If he likes photos, you could take photos and label them - put the photo in the centre of the page and have two strips of pictures to describe them down either side. Some resources here: www.tulareselpa.org/Autism/Downloads.shtm
Again, depends on what motivates. Motivation is everything. What can you think of that he really, really likes and is happy to communicate for? What does he ask for vs label (mand vs tact)? These are the ones to start with.