Flu and Antibiotics

I'm not sure about the exact name of this, I'm talking about when you have a very bad flu and it goes into your sinuses. Doctors are very keen on prescribing Antibiotics for this.

My brother is just in this situation and I'd like to know first of all what is there to be done with natural methods to prevent and treat this problem and what alternatives are there for Antibiotics , and secondly, now that my brother has already been given Antibiotics , what is there to be done to restore the balance in his body before any unbearable symptoms appear?

Please take the time to give us a hand!

Jo