Boy's/men's ballroom clothing is generally basic black. True stretch, body molding Latin pants and the true Latin shirts that snap at the crotch can be expensive as they are tailor made. At 9 your young man is still growing. It might be less expensive to shop for black slacks and a black shirt for him to dance in. If you wish you can embellish the shirt with crystal stones to punch it up. I would save a little money and order percosa stones. They have a lot of sparkle on the dance floor, but don't cost as much as the Swarvoski stones. I like ordering my stones from The Rhinestone Guy, he sells it all. http://www.rhinestoneguy.com/index.html
You might try E-bay typing in Men's Ballroom.
Be sure to try Used Men's Ballroom too.