This is a good question. The best way to keep your little son's nose clear is to use a saline nasal spray which you are already doing. Children of this age do not usually have sinuses that are fully developed yet so it would be unusual for your son to have sinus problems. he does not seem to respond well to the usual treatments for a persistently blocked nose in a child. This may be due to either enlarged adenoids or to nasal allergy (allergic rhinitis) He should be seen by a paediatrician or preferably a paediatric allergist so that the cause of his blocked nose can be properly diagnosed and treated.
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