6 year old child has sudden onset of fever and rapid breathing. What might he have caught?

Many causes. Any cause of fever can increase respiratory rate. This could be a sign of pneumonia or other serious bacterial infections. Fever and shortness of breath or difficult breathing is an even more concerning set of signs. The significance of these signs is dependent on the height of the fever and the severity of disturbed breathing. You should discuss this with your health care provider.
Many things. Without more information it would be very difficult to even make a guess as to what is going on. If this child is having any other symptoms or actually having difficulty breathing i would recommend that he be seen by a family doctor or pediatrician asap.