Thomas was born with a mark on his right arm that at first they thought was just a birthmark.
However after further testing it was revealed he has a malignant tumour on his kidney, and his birthmarks were also from his cancer.
He also has tumours in his lungs, and in his leg bones.
Speaking with Daily Mail Australia the couple say they think William understands that his little brother won’t survive.
'He is aware that Mummy and Daddy are very sad because Thomas is not well,
'William also said that maybe the doctors can't help his baby brother, that's why Mummy and Daddy are sad. So we think he knows in his own way.'
Despite knowing their boy will be leaving them soon, the family say he has already taught them so much about what’s important.
‘We often get caught up in our own little world of first problems and we sweat the small stuff, or complain about things that don't matter,
'We forget how to be grateful of the things and life we have. We must live with a purpose, gratitude, and magnitude of love.'