In the meantime, I try to find pictures of people that are 'acceptable' physical substitutes - chosen for the sole purpose of giving people the idea of what I imagine them to look like.
William has a nice body and the men that surround Talon, his friends and fellow-prisoners, will all have nice bodies - a forced situation thrust upon them because of who they must become - Sons of Ammon, leaders of armies, etc.
The picture of William are how I picture Talon at the end of the third book, Aftermath, and into the fourth book, Sacrifice of Souls. At the time, he is only 16. Here's the reason it will work: Talon is seven and a half feet tall before he quits growing. Of course, William will not remain young - except in pictures.
Most characters mistake Talon for someone older because he is so tall and strong. Talon carries a big load on those broad shoulders and by the beginning of Sacrifice of Souls (SOS) the load gets bigger, a lot bigger.