See his pediatrician
This sounds like croup
, a viral infection of the throat including the voice box.
It is not life-threatening and usually will resolve on it's own. There can be more serious complications and so its a good idea to have your son's pediatrician examine him. If it is more serious then there are medications that will help him recover.
http://www.Youtube.Com/watch?V=bf5sxehpgck to learn more.
Answer assisted by Chethan Sarabu, Medical Student