Aiyoh I understand lah, this is stressful! A few of us mums went through this too, first thing is to check with the doctor to make sure there's no underlying issue, sebab sometimes there's reasons that need proper attention. Once doctor gives the clear, try offering calorie-dense foods like peanut butter, avocado, eggs, and full-fat yoghurt in between meals, and make sure feeding times are consistent so his tummy gets used to regular meals. One of my mum friends swears by adding a bit of oil or ghee to his food and giving him healthy snacks like banana with kaya, made a real difference for her son!