In the end, Zhao Jing went home to change into his own clothes.
Chen Song's clothes were too loose for him, which was one reason. The second was that men generally didn't own many sets of clothes. He had already borrowed one of Chen Song's outfits and was worried Chen Song would have nothing to change into later.
If De'an had been a bit taller, he might have just worn De'an's clothes. But De'an was shorter than him; wearing his clothes would have left Zhao Jing's wrists and ankles exposed. That was why Chen Wanqing had brought him Chen Song's clothes when she came back to get him an outfit earlier.
Zhao Jing had another reason for going home: to let his mother know he would be staying at the Chen family's home for the afternoon. That way, she wouldn't worry about his safety. He promised he would definitely be back before dinner.
Once that was done, Zhao Jing returned to Chen Wanqing's house, holding an oil-paper umbrella against the rain.
