Welcome to FT ^ During the SIN stopover, your son will disembark into the terminal before immigration and customs. As long as he stays airside (doesn't go through immigrations, which he doesn't have to), he can wander through the terminal. How old is he? The airport has great shops, restaurants and lots to look at.

Before boarding, he will have to go through security. This is handled at the entrance to the gate.

PS - I didn't think BA flew to MEL anymore? Off hand, I thought BA 11 went to SYD?