You won't find an official answer, and the trials in children are phase 2 and not scheduled for completion until August 2017.
So let's resort to the logic of first principles.
Children are different in three primary ways (physiologically)
- They are growing both physically and mentally
- They may not have all the metabolic pathways for drug breakdown seen in adults
- They are a different weight
He's old enough that most of his bits are fully formed but some - cartilage in bones allowing them to grow, and his brain are not - it's possible these medications could do stuff here - ?
You can observe that for many common medications there are < 12 yo doses and then Adult doses - tick
If your son is full sized then we need not have concerns that the tablet size is too big for his body - tick
So if you weigh up the risks of treatment they are real, and to a degree unknown.
Then you look at the impact of Hep C and the risks of having it - they are real, substantial and well known.
To my logic the risk of treatment are small, the risks of not treatment are large, so the risk:benefit stacks up on the treatment side.
He's around the age of Gillick competence which is where the child is deemed capable of consent (usual situation an underage girl seeking either contraception or a termination).
On that basis if he wants to do it, and you are assessing a significant impact treatment is not unreasonable.
There is nothing magical that suddenly happens on a child's 18th birthday that says yesterday treatment was dangerous, but today it's safe. It is an arbitrary line in the sand and an adult sized 15 year old should tolerate an adult dose of medication fine and have a risk profile similar to an adult.
I would certainly be thinking along your lines if faced with the same dilemma.