Aaaah! How do i sing better?

I have been cast as Freddie Mercury in the school musical and i have to sing some notes in the Falsetto register (somewhat unsurprisingly) ;) . I can hit the notes pably, but the problem is that as soon as i go into Falsetto, my voice goes really quiet. It'd be great if I could give it some real volume on the high notes, like Gary Barlow off of Take That... is there any way of doing that or is that his modal register? Thanks :) Im a low tenor naturally btw