Hi! What wheel did you buy? a flying saucer or an upright, and what brand? An 8" is fine if he can actually turn it, if he can't make it move, it's too big, but it doesn't sound like that's a problem. There are some wheels that have a front piece, like The Wodent Wheel, (see links), or the Silent Runner (see links), which would stop him from flying off. If you have a flying saucer, you also need an upright wheel, as a flying saucer can cause back injuries.
Links:
Silent Runner:
Silent Runner 9", Exercise Wheel and Cage Attachment for Hamsters, Gerbils, Mice and Other Small Pets - Walmart.com - Walmart.com
Wodent Wheel:
Wodent Wheel 8" For Small Mammals - Exotic Nutrition
Note: The links I provided aren't necessarily the right size they're just to show you the different types of wheel.