A doctor can write a prescription for any patient, regardless of age. He may have meant that your son doesn't fit the criteria for being helped by the drug. For example, Tamiflu is only helpful if started in the first 48 hours of the flu. You don't want him taking a medicine that won't help, and drugs become less effective when given to people that they won't help.
Not all people can get Tamiflu. I would think you can google it to see what age you have to be. I'm sorry I could'nt be of more help. I do think there may be an age limit on Tamiflu, Could be wrong about that though.