Thank you for your query.
Viral upper respiratory tract
infection take their own time to get controlled and I can understand your frustation and the child's repulsion to medicines. You did the right thing by adding Benedryl
which shall help control the cough
and the secretions. Continue giving him the medicines three times a day and if possible steam inhalation to liquify the secretions.
By mouth hurt are you suggesting that you son is complaining of pain in the back of the mouth (throat). This is very common in upper respiratory tract infection. If the pain is somewhere else then I suggest that a re-examination by the doctor will be in order.
Since an antibiotic is already being given you should wait for another day or two before considering to change the antibiotic.
I hope I have answered your query to your satisfaction.
Wishing your son a speedy recovery.