Pornchok, 27, entered the BC4 boccia final against Samuel Andrejcik, 24, having beaten him 4-1 in the group stage. However, Andrejcik turned things around with a 4-0 victory to take the gold.
Pornchok won the bronze medal at the 2016 Paralympics Games while Samuel won the silver medal in that competition.
Pornchok will compete in one more event, the mixed pairs BC4 boccia. He and his partners Ritthikrai Somsanuk and Nuanchan Phonsila will face Colombia in Pool B on Thursday at 9.25am.
Pornchok’s silver was Thailand’s ninth medal at the Games.