Arhu indeed has a dialog option to complete the King's Summoner Quest. Yet it will not be immediately apparent as you start talking to him. You will have to select the option on source hunter's business (an option you had right from the start) and then talk about Braccus and the Baron (which again you already did if you have the King's Summoner Quest in your quest log). But this time Arhu will acknowledge that raising Braccus was Telyrion's doing. You get some xp and +1 Rep.
Btw. you can also go to Arhu to complete this quest immediately after you loot the undertaker's hut and read the diary.
