That's why he wanted Chen Yao's help.
Although Chen Yao is currently collaborating with him, the Old King doesn't know that Chen Yao and Shangguan Xiu are cooperating.
Therefore, it would be easier for Chen Yao to enter the Imperial Palace and find an opportunity to take Arno away.
Also, Grandma An is currently staying in the Royal Hospital of the Royal Family of Country K, which is not far from the Imperial Palace. However, ordinary people can't enter there; only Royal Family members can.
Furthermore, there are many people taking care of Grandma An around the clock. If Shangguan Xiu went there in person, he could handle those people, but it would be very troublesome. Since Chen Yao is a Royal Family member, it would be most appropriate for him to go, and it wouldn't arouse any suspicion; no one would suspect he's going to take Grandma An away.
On the other hand, if Shangguan Xiu went, he might inadvertently attract attention and get caught by the Old King.
