Thank you for your query at DoctorSpring.com
I understand your concern.
Since he is currently not on chemotherapy, and the last one was 4 months back, you can give him Syp. Ascoril ( Guaifenesin ) 2 teaspoons 2-3 times daily after every 6 hours, if required. It won't cause any harm, and it won't lead to any kind of drowsiness.
If syrup is not available, Tab. Guaifenesin can be given 400 mg orally 3-4 times every 6 hours, if required.
You will need a prescription for this medicine, so kindly get one from your local physician.
Please feel free to discuss further,