What kind of sports? And ~ what kind of party are you looking to do for him?
You could always take the route of doing like an olympic games. Make the invites with the rings or something, and then for activities hold "sporting events" (basketball game of horse ~ or something shorter, races, baseball throws, jumping contests, soccer goals, etc...) and then give out fake medals at the end...
Or another idea, instead of trying to make the invtations look like sports balls ~ make it look like a field. If he likes football ~ make it a football field, basketball ~ a court, swimming ~ a pool, etc... Then you can shape the cake like the ball if you choose.
I know you were looking for just invite help, but you could do sooo much with them, and then co-ordinate the rest of the party to accompany the initial invite...does that make sense??
Good Luck, I'm sure you'll rock Shannon (look at your letters

). Let us see what you come up with when you are done!