chronicrunning nose & then two days latter cough starts & post nasal drip in toddler of one & half year boy

my boy sneezes every morning & evening.his one of the nostrils is blocked like inflamed in nostrils.we can't take him out in wind or dust or smoke or we can't bath him daily as he get cold very often.whenever we do this nearly evening time he starts very frequently sneezing & then almost one hour latter his nose starts running ( discharge is thin watery clear but continuous)then next day discharge from nose starts thickend & cough starts & left gland inflamed,first he cough startswith phelgum but after giving nebulization dry cough starts in a sequence.mostly in night evening & morning in sleeping probably due to post nasal drip & strep throat (itching in throat),most of the day his voice horse please help me with remedys.i give him spongia tosta 6 for a while but does not seem work well.