DS is just coming up to 18m, and doesn't really have any words yet. He makes some sounds fairly consistently - mainly doh for dog, and he attempts to say moon, milk and more, does a bit of mamama sometimes, and does a hissing sound for snakes. He also has a couple of other sounds that I am pretty sure are supposed to be words but I haven't figured them out yet 
To be clear I am not worried at all about him (although a lot of his friends seem to have a lot more words than this!!) He understands loads and can follow some quite complicated instructions. He LOVES being told words, eg at dinner I spend the entire meal going plate, fork, pasta, beaker, table, pasta, fork etc etc etc as he points at everything over and over and wants to be told what it is. I just can't seem to get him to even attempt to try and say any of these words. If he points at something, and I say 'you do it' or similar, then he just looks at me and points to something else. Also, I am very good at interpreting what he wants based on a point and a few eh eh eh's (this is the sound he makes constantly!) so he doesn't have to try and say anything to get me to understand, not sure if this may also be hindering him.
Does any one have any suggestions for how I could encourage him to have a go at making some different sounds and trying to say words?
Thanks 