He might be having Eustachian tube dysfunction because of cough and cold or he might be having wax impaction. You need to get his ears checked by an ENT doctor before you change hearing aid or take any such measure because cause needs to be known first
Next Steps
Take cough syrup (syp rapitus 10ml twice a day) to treat the cough. Rest of th medication to be started after examining ears.
Health Tips
Take steam inhalation from nose