HI, thanks for using healthcare magic The mucinex is a mucolytic and would help with a wet cough or decrease the thickness of nasal secretions. He can use this with the zyrtec or the benadryl but he cannot use the benadryl and zyrtec together because they are both antihistamines. If the antihistamines are not enough then he may need medications such as ketotifen or montelukast. These work at different points along the allergy reaction. It would be best to find out the exact source of his allergies so that his exposure can be reduced. This can be done by allergy blood tests or skin prick testing. He will continue to have symptoms, even though they may be less, once he is repeatedly exposed. I hope this helps