We finally have a diagnosis!
The day after GL turned sixteen, we finally got a diagnosis. He has a chromosome 17 q12 x1 dn microdeletion. This doesn't change the fact that he has autism, but 100% of males with this deletion have autism, so the genetics clinic considers the chromosome disorder to be his primary diagnosis, and autism to be a feature of that disorder. He was part of the first study linking this disorder to autism, but now there is enough data in to confirm it.
This doesn't change our expectations or plans for him, but it is strangely reassuring. In his particular case, we know that autism is the result of a missing gene, not an environmental factor. It wasn't anything we did. It is also a de novo deletion: we both have the piece he is missing, but God chose to leave it out when making him (some would call it a random mutation) so he didn't inherit autism from us. That also explains why his brother does not have autism, and relieves our fears that this was a factor in any of the miscarriages.