Thanks for your query. I am afraid that it is not possible to diagnose just by the temperature and the colour of the tongue. I suggest that you get your son examined by a doctor who shall take a detailed history, do a complete physical examination and investigations, if required. Then only a diagnosis can be made.
White coated tongue can occur if he is a mouth breather or even in conditions like typhoid fever
. Since you say he has a cough
then may be he is having blocked nose and must be breathing from his mouth while sleeping.
Wishing your son a speedy recovery.