Polyuria, polydipsia and weight loss point to something serious

Jackson, a 24-year-old male, presents to his GP with a four week history of worsening polydipsia and polyuria.
He reports he has also lost 20kg over the preceding two months, without trying to.
He has seen other GPs twice in the prior two months, as he was worried he had a UTI given the increased urine output, but nothing grew in his samples.
He has no significant medical or family history. Jackson is a lifelong non-smoker with only occasional alcohol intake.