Don't they usually babble around 6 months? I should know this because he isn't my first...
He has jsut started babbling when he is grumpy/tired. Sounds like he is having a good old moan as he mutters away "mumumumMU!" "bububBAD-DAD!"
He only does it when grumpy with none of the happy babbling I remember from my other children.
Is he actually babbling or just complaining 
On the other hand he really clearly said "Very pooopy!" the other day. I checked his nappy and he wasnt.... 