I have to tell you - I was in the same spot. Not only was it hard finding child's fanny packs, but even more difficult for a BOY (my son is 8 1/2 too.) I found some really girly ones, but forget that. Found toddler ones at Ames which hold water bottles, but the carrying room was very small and waistband too small for my older son.
So, here's what I did and it was perfect: I just got him an adult size. Of course it was too large in the waist. However, I can't really explain how I did it but I'm sure you can figure it out - I adjusted it as small as it would go and then somehow doubled the strap over even more and fastened it with a safety pin. I was doubtful, but the safety pin held up for all 8 days on a very active boy. I would recommend this above all because the adult-size fanny packs are more roomy than kids size and, as you know, easier to find.
I got the water bottle-carrying toddler-size fanny pack for my 5 year old (Ames, Fisher Price, $2.00) but the water bottle took up too much room, compromising space in the fanny pack itself and it fell apart 1/2 way through the trip. I just carried one big water bottle w/a strap and we shared - hey, we're family.
Good luck.