if he can call the cat dada then he can speak. My son calls any man daddy in the cutest way which always makes them look a bit wary lol
He still cant say mummy and has named me muma which is fine for me.
He did speak for a while at a young age and when he learnt to walk (early at almost 10 months) he stopped saying words and just babbled for a month or so.
As i said before, hes probably more interested in walking running etc to worry about words, especially as you can understand him and what he wants anyway.
Im sure hes fine but if you are really concerned then speak to your health visitor.
Have you tried the ivillage mums boards? the september board is
http://messageboards.ivillage.co.uk/iv-ukpbsep t05 I use these boards and have done since i found out i was pregnant. it may be worth you joining and discussing the matter with mums of babies the same age. You wont find yourself alone and may even be able to help other mums.