The only thing that can be done for lockjaw as far as I know is to increase their vitamin intake and food variety and force feed them through it. A local vet might have the neccessary tools for force feeding. Since its a Pufferyou could also be seeing an issue with dentistry since their teeth can get so big it prevents them from being able to eat. It could also just be a vision issue where they cant always see the food. If you ty force feeding and they clamp down on the feeding device, then it isnt lockjaw.