Do you speak baby talk to him?
It was pretty recently proven that baby talk ("motherese," the professionals call it) is GOOD for a child's speech development. It breaks down the big, scary grammar points into something a child's "Language Acquisition Device" (a theory by Noam Chomsky, not necessarily proven) can handle.
Some children do acquire speech slower than others. The babbling is where he's working out usable phoneme strings ... ie, trying to see what sounds put together in what ways will get your attention or get a reaction.
Does he know his toy's names? Like a teddy bear, or a puppy? It's a good place to start to encourage him.
I doubt highly that his siblings are "talking for him," and I don't really see how it would work.