Yeah, that's one of the reasons why I think the Asperger's diagnosis needs to go bye-bye, and fast. What do you do when somebody scores a 69 on the IQ test and has every single characteristic of a classic case of Asperger's? ...PDD-NOS. Lovely, folks. Really useful diagnostic criteria there.
There are two things you might call an "average IQ": 100, which is the smack-dab center of the normal distribution, and the range of 70-130, which is two standard deviations away from normal, or the "normal range", within which a person can be considered to have an IQ which isn't particularly unusual. If you're talking about "below 100", then half the people in the world are in that category, and it really only means the person's in the lower half as far as IQs go.
If you're talking below 70, then yeah, you're not supposed to get an Asperger's diagnosis, but like I said, I don't like that criterion at all because what do you do with the people whose cases tick every check box but the "no developmental delay"? They're every bit as Aspie as the people who scored better on the IQ test. Below 70 is what you call "mental retardation" (though only if there are also problems with everyday daily-life type skills--if there aren't, you can't diagnose mental retardation), and in general you expect slower development, but when autism's in the picture all bets are off because we've got just plain weird development in general.
An IQ of 65 is at the top end of the "mild MR" range. If you had that score and you weren't autistic, the prognosis would be something like, "Will finish vocational high school program; will live independently; will probably support self with semi-skilled or unskilled work;" but with autism... who knows? You just can't tell at five, especially if there's a communication issue. Think about it: How do you know how to do the test? That's right--the tester gives you instructions. And what is that? Communication. See the difficulty? That's why testing autistics is ridiculously hard and notoriously inaccurate.
And may I note that IQ test scores don't even say much about autistic intelligence. Anbuend noted how much her scores have changed over her lifetime; I know of others whose IQ scores started out low and got higher, or have been all over the board from MR to genius; and a lot of us can tell you how our scores are pretty much invalid for us, too. Either you get a low score because some overall weakness was creating problems that didn't let you do the test; or you get a high one that doesn't mean much because you're being held back by something that wasn't on the test; or you get a completely meaningless one because your subtests have such a wide scatter that your strengths and weaknesses have nothing to do with your full-scale IQ.
If I were you, OP, I'd ignore the IQ test, look at the subscales for what information you can get about your kid's specific strengths and weaknesses, and generally approach it as an individual person who isn't defined by his IQ or by anything else. You just can't define a person's ability to learn, let alone an autistic person's, but a simple number.