Hi..
Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern...
As per your complain and presentation of symptoms tongue can appear white in different ways..
In case if the tongue is appearing pale and whitish it can be due to infection and weakness while if there is a cheesy white coating on tongue then it cannot be due to sinus
drainage but it will be due to yeast infection known as THRUSH..
It is true that cough and fever are signs of viral infection and possibly you must have given antibiotics to your son after advise from your doctor and many a times antibiotics intake can lead to thrush because antibiotics can disrupt the normal bacterial flora of the mouth along with harmful bacteria..
In the absence of resisting bacteria against infection
fungal infection proliferates leading to white coating on tongue..
You should consult a Paediatrician and get him evaluated and a microscopic examination of the small portion of the white coating can be done microscopically for confirmation of the cause..
If thrush is confirmed
Nystatin mouth rinses,
Clotrimazole lozenges and taking more of probiotics like Yoghurt will help..
Hope this information helps..
Thanks and regards..
Dr.Honey Nandwani Arora.