The overt purpose, of course, is to hope that Xiyun and Dongqing can continue to maintain friendly relations. Now, Dongqing is even less of a match for Xiyun, and he doesn't want Dongqing to fall out with Xiyun before he becomes Emperor.
He wants to wait until he has the power to contend with Xiyun before falling out with them, and possibly even destroy Xiyun to incorporate it into Dongqing.
As for the ulterior motive he's secretly harboring in his heart…
Since his last mission to Xiyun, he hasn't been able to forget An Jing. Now, he's here also hoping to see An Jing. However, An Jing is now the Queen of Xiyun, equivalent to the mother of Xiyun. He can't just go to the Xiyun mansion looking for her like last time. He has to be more discreet to avoid his intentions being discovered and ruining the relations between the two countries.
To conceal his intentions, Dongfang Tianyang doesn't dare to mention Queen An Jing on his own.
