V.W.
As long as he understands when YOU speak to HIM, then you have a lot more time before you need to worry about talking to the pedi. Our kids were early talkers (in the 10 month range when words --or their version of them--started appearing.. actually our daughter was 7-8 months when she started saying something that over time morphed into her brother's name...we just didn't realize at the time that THAT is what she was trying to say: a forceful "U-n!" looking at her brother became "Urn!" became "Ur-in" became "Aaron"). But again... our kids were early talkers. Your son might very well be saying something that counts as a word, you just don't know it yet. ("words" for babies are anything that they use to mean something specific... it doesn't really even have to sound like anything recognizable--as long as it has meaning: Bah! could be "bottle" if he is using it with meaning, for example).
Give it a bit more time. I know lots of kiddos that didn't say any recognizable words until almost 2 years, who are absolutely FINE and NORMAL.