This character is Sirius, one of the main characters, as a child.  In flashbacks, he is shown as a child, probably around 10 years old.  He is a bit on the quiet side and quite mature for his age.  

Preferred voice: A typical younger boy's voice is fine, but more serious and mature tone-wise.  A British accent would go well since the VA for his adult-self has a snobbish-British accent, but this is not required.