I went with the Iowa Test of Basic Skills (ITBS) at-grade test the first year, just to be sure that he passed...and boy, did he pass!
It was SOOOOOO easy that it was almost painful to administer. The hardest part was keeping quiet when he did something profoundly stupid because he wasn't paying attention or didn't follow directions or something. He should have had a perfect score on everything except social studies. He didn't, and it was only dumb mistakes that made him miss anything. Perfectionist mom had to REALLY bite her tongue!
Actually, I'm also debating with myself right now about whether to grade skip him a year in the testing. Part of me thinks that's sort of dumb--it locks us in to a grade skip that I may not want him to have down the road strictly because I'm bored with the test questions. What if he doesn't learn the times tables this year? Is he going to be ready for 4th grade math? (Sure, he's done some algebra and geometry...but that won't be on the 4th grasde test!) One grade skip on the testing isn't going to provide significantly more challenge or raise the ceiling enough to tell me anything, but it does mean that I'm declaring him a 3rd grader to the school system and we can't back up and take the 3rd grade test again next year.
On the bright side for you, I'd say the testing was super easy. It was not a hard test in *any* sense. If your son would be grade-skipped in school and you think he'd pass with flying colors, I don't think he's likely to fail the ITBS.
It's just that darned future unknown and asynchronous development thing that's got me scratching my head.
