Helping my son get allergy relief until medicine kicks in?

my 4 year old son has allergies really bad. he is starting back on his medicine for the spring through the fall. it takes a few days to get any relief and he is coughing so bad he barely gets to rest and he is puking from all the drainage (this is not uncommon for him). i was wandering if anyone knew what i could do to try and help give him some relief from the draining and coughing until his meds help. i have a humidifier in his room already. please help. thank you.