My son will not talk. He just refuses to say anything. He will use the sounds of things instead of words like brum brum instead of saying car. He hears perfectly well. He is one of three children so has plenty of stimulation.

I think this is a case of won't talk, not can't talk because he will say a word then not say it again and says no and shakes his head when we talk to him and ask him to repeat the word back to us.

I'm so worried because he is due to start nursery and he will not even be able to ask for a drink. He seems so babyish. Help!!

Many very successful, intelligent people didn't talk much or at all for this long or even longer. But without testing (and they will want to double check his hearing), you won't know if he's just 'holding back' or has something wrong developmentally. You could start looking into where to get tested while letting him adjust to preschool, and see if he changes in that environment. If he can make sounds for things, he can do so when he's thirsty. And it might prompt him to use words if new people around him don't know what he wants, and he might be more willing to be like kids his age as opposed to his siblings.
Getting tested sooner rather than later does have advantages if there is something wrong, like autism, because early treatment is more successful than later.

I am sorry you are going through this... My son was diagnosed with autism.. It took him awhile to be able to talk and to be able to talk plainly. He is 9 years old now and within the past few years really started talking. He started making sounds, but wouldn't talk. You should get him tested, there are programs out there... really good ones.. Autistic children can be a handful, and will take up a lot of your time as will any child with a special need.. Like I said, my son is 9 and I am still going through a lot with him, but he is so worth it. God chooses who he lends the special children to... and whoever he chooses is very blessed.