AnneMaria, I might be speaking out of turn here, sorry if I offend - does this child have a special need? Perhaps any profesional they come into contact with can offer help. Otherwise most children start off with pretty short attention spans and its up to us to 'lengthen' it. Sitting down with them at the kitchen table playing an age/stage appropriate game for slowly increasing periods of time. Reading sessions, craft sessions etc etc. Children cant learn to 'amuse' themselves, as our parents used to call it, unless the adults in their lives have shown them how.
On the otherhand I've always thought its a bit much expecting very young children to cope with travelling in cars on their own and whenever its been possible an adult has always sat in the back seat with them. Its usually me and for the GC I have weve used things like 'fuzzy felt' lift the flap books, appropriate story CDs (Oh the memory of travelling down to Devon one year playing Fantastic Mr Fox over and over). 