Andrew Gallagher says his brother's death has "ripped my heart out".
He made his comments on a social-networking site where hundreds of people have been leaving moving messages this week to remember the much loved teenager.
"I have no idea if I'll ever mend. I know I won't. I wish this wasn't real. You are such a beautiful human being. You were loved and adored by so many," he wrote.
He thanked the public for their support and praised the many who have signed up to the "RIP Josh Gallagher" Facebook page.
"I would like to thank everyone, almost 3000 of you, for your strong, and powerful show of support for my beautiful younger brother, Joshua Gallagher, and our family. Your prayers and thoughts thus far have been received. Your support for us through this dark time is needed."
Mr Gallagher revealed details for Joshua's wake and confirmed that his funeral would take place on Monday (October 22). Between Friday and Sunday friends only are invited to pay their respects.
"From 10am until late we would like to invite friends of Joshua and our family to pay their respects during the wake. Whilst condolences are in order we would like to celebrate Joshua's life and how he inspired others to feel great about themselves."