Your 10 year old is just the audience for Ben 10. My son also just turned 10 and loves it. And since they've made him, cousin Gwen and Keven into teens, its great for tweens. If you're going to get him some Ben 10 toys, or he has some that are clean, put 'em on the cake. As for invitations, find a Ben 10 image either plain Ben and use it on the invite or see if they have anything onilne you can down load. Or you can scan or download your son's favorite Ben 10 alien and put his face on the alien, something like that. Apply the same for Go Diego, Go! party. As for party idea, find soemthing in the show and use it in traditional party games. But the images on Bingo "cards" even. They usually also have Ben 10 games and things that you can purchase. They'd be fun. Get silly string and cheap laser guns for Ben 10 party.
It's not rockets science, although if your 10 year old is like mine that's space science is what he's interested in. Quit making it so hard. It's really pretty easy if you've got a computer. Or if you do a decent freehand drawing, apply the same only put your kid in the face, etc....