Pediatrician from pune. Pls help.

My kid - 2.5year male child Height -93cms Weight- 12.3 kgs Issue- severe lack of appetite, severe constipation resulting in very hard Stone like stool and vomiting. It started after a pediatrician gave him very high dose of antibiotics for a fever due to 'hand-mouth-foot' disease. After that illness in jan/feb 2023 my kid has started very bad digestion issue and has stopped eating. Due to fear of vomiting and constipation he has stopped eating, he gets nauseatic and has become very week (pls see photo) he'll demand milk only. We have obviously reduced milk to bare min. But that has not helped his weight gain or appetite infact he has lost weight. He is visibly become very week, skinny and has become lethargic. He has reduced interest in playing. Pls help.