you should not be worried for the loss of voice your child is having at present as this is a sign of laryngitis
and is viral in almost all of the cases. and most of these infections are self limiting and resolves within few days to may be a week.
meanwhile you can give symptomatic treatment for cough and fever
that your doctor must have prescribed. and wait for the infection to resolve. your son will regain his voice as infection starts clearing.
but as you said coughing is becoming more troublesome for your son, you should get him evaluated by a pediatrician
for any congestion in chest.
some danger signs you have to watch for in any respiratory infection would be breathlessness, stridor
or excessive irritability, high grade fever, not accepting feeds bluish discoloration of nails or any part of body etc, in which case you should rush to an emergency.