Hayley,
Thanks for your question from Australia (I think?)!
So no, you cannot have a magic band hold more than one person's tickets, but I don't think you should worry. I speak from personal experience. My youngest has tiny little arms, and she has had the bands from both the
Disney Cruise Line and the Walt Disney World
MagicBands slip off a few times.
But while the ticket is technically linked to that band, it is more importantly linked to your
My Disney Experience account. If he loses the band, Guest Relations can void out that band and tie all his information to a new band quickly and efficiently.
One more tidbit you may not be aware of. The bands have a tear away section that can leave the band smaller. That makes it easier to fit onto tiny little wrists. So while the standard band may seem too big for him, when it is adjusted it is likely going to be fine.
Have a Magical Vacation!