Ok this is just a theory and a very loose one. But I'm wondering whether DS had diabetes. Mainly due to his constant extreme thirst, unable to gain weight, natural aversion to sugar, lethargic after bursts of energy.
Tbh it's the thirst hats getting me. He's 4 and must drink 2.5 litres a day, he will be fine then scream for water.
I can hardly get a cup down my 7 yo