On the day of Julius' 18th birthday, while the real prince was sleeping so that he could recover from his battle on death island, Jun had already cast an illusion to make himself look like Julius so that he could take his place during the global celebration.
The only people who knew of the state Julius was in were Henry, Roger, Ronald, Jun, Liz as well as Julius' biological mother. Other than those people, no one else knew about the state the real Julius was in. Even the kings and queen along with the captain and vice captains of the royal guards from the other kingdoms had no idea and were simply told that Julius had already recovered from his injuries due to his great status points for each of his aspects.
After Jun and Julius had their conversation on the illusion plan, Jun had went and told Henry which was a relief for the king as he didn't want to ruin the reputation and good name of him along with his son.