Has he had a cold lately? The mucous he is vomiting could also be from it running into his tummy. This is not uncommon with children who have colds.

Another way to break up his chest is to pop him in the bathroom when you have a hot shower (keeping doors and windows closed) - the steam helps break it up.

I agree with Sammie about chest therapy... face down over your knees and gently hit his back with an open hand. Start at the base and work up - you need to cover the entire back. Think of it like playing the drums... you want it to be firm enough to break up anything on his chest but not hard enough to hurt or cause harm. This is a great thing to do after he has been in a steamy room.

Hope the GP is able to help you some more. Let us know how you go.

MG