my colleague told me it*s now a mandatory for everyone to wear seat belt when taking the backseat. is that true?

It's true. Riding in the backseat of cars without wearing a seat belt is punishable in Jiangsu province starting from November 1.

Those who fail to wear seat belts as required while the motor vehicles are running on expressways shall face a fine of RMB20 yuan, and those who fail to wear seat belts as required while the motor vehicles are running on roads other than expressways shall face a fine of RMB5 yuan, according to relevant provisions.