Has he been checked for nasal polyps? More usually, people sniff and sniff due to the post nasal drip it causes, but it could be that it irritates his throat making him clear his throat constantly too.
It could be a form of nervous tic also?
Also have him drink more water.
Did he have allergy testing done to see? Some allergy medicine may not work, he may need shots. I know it doesn't work for me.