My DS is 18 months too and has really only started to get a few 'real' words in the last few weeks and still only 8-10. I was also starting to wonder a month or so ago and then one day he said bye bye to DH out of the blue, from there he's really started expressing himself with 'no' and 'uh oh' and is trying to mimic words. So I wouldn't worry, from my experience it will just start one day and slowly develop from there.