Stuart is one of the three Ranger apprentices in The Kings of Clonmel, where Will is assigned to administer an exercise to Stuart, Nick, and Liam that required them to use strategical skills to sneak into a fortified, fully guarded castle. The three boys, who were in awe of young Will's accomplishments, tred to copy the tactics he used in The Siege of Macindaw to successfully break into the castle. The three suggest that they can summon Skandians and plot an attack on the castle, an idea near identical to Will's own tactics. This grieves Will to no end because they are not using the tools given to them to complete the mission, but are instead copying him.
Presumably, Stuart successfully graduated from his apprenticeship and was assigned a fief, currently working as a member in the Corps..