The boy from Oxted in Surrey won the inspirational young person 15-18 years award - and Harry was told he had a younger brother when the royal asked about siblings.
'Does he drive you mad?' asked the Duke, and Declan replied 'no, we get on fine' - and Harry quipped with a smile 'you know what - siblings'.
When told the 17-year-old and his brother went to the same school, the duke said 'you're at the same school, that sometimes makes it more challenging
From the WellChild awards- can he put anyone first?