Interesting as at early level there are broadly tow categories, namely requesting and commenting.
For example, a child might say or sign 'plane' and it might mean 'OOh, look, there's a plane! How interesting!' (comment) or 'I want that plane there right now (request).
Generally requests come a bit earlier than comments but in a child with disordered communication,it can be a bit topsy turvy. Obviously we want them to do both.
Miss, can you engineer a situation in which you tempt/entice him to request something? For exampl,e play with something really interesting near him? As soon as he shows mild interest of any sort, model his little hands to make the relevant sign (or get someone else to do it while you carry on tempting him and overracting madly)then give it to him immediately, praising like mad.
Sometimes we unintentionally tend to quickly to our children's needs which reduces the need for independent communication attempts.
PECS people call it 'waitress service' which is a great way of putting it.
Write it on a big sign and put it on the wall tyo remind yourself not to. Obviously you do not want to withold food or drink or basic things unless he asks (esp. if he doesn't for these) but you can engineer loads of situations to get him to request:
-close a favourite dvd in its box and let him struggle with it a bit. As he loses patience, jump in and get him to sign 'open' then do instantly, praising madly
-'forget' to put a spoon/fork with his food
'accidentally' pull the plug on a favourite CD/DVD
-'accidentally' close the door on him as you leave the room
-'accidentally' put a favourite toy out of the way
-'accidentally' forget ot do up his shoelaces
-'forget' to push him on the swing
Get him to think and problem solve and be in a position where he has to ask for something specific or the condcept 'open' 'close' 'up' 'down' 'more' 'finish'