Is it safe to give a 3 month old infant this medicine without doctor's knowledge?

I am thinking of giving my three months old infant (turning 3 months by February 16,2006) Cardec DM Drops. He has a week old of colds and cough. I have tried to call his Doctor, who is new to us, but the only person I have talked was a Nurse Practitioner who won’t write a medication to give to my sick baby and insists on doing conservative managements like cool mist humidifier, suctioning, etc. Which really doesn’t help much. I don’t want to wait till my baby have complications from this colds.

My question is, is it safe to give it to him without the knowledge of his Doctor?

Thank you for your time.