Looking at the SB-5 results, I noticed that something called "Knowledge" was his lowest score by a mile in both categories, and that you'd said you'd been a laid-back parent with regards to early education (and lest you think that anything that follows here is a criticism of that, let me jump ahead in here by saying I feel that's a great approach for that age). I'm no expert on that test by any means, but I wondered if "Knowledge" measured crystallized knowledge, whereas the "Fluid Reasoning" scores (which were very high) measured things any child would have to figure out. That would suggest to me that the Knowledge sections are largely influenced by how much exposure he's had to formal education, which you've indicated is very little. The scores on other sections would seem to me to be more predictive of his ability to absorb crystallized knowledge, given more opportunities.

So I googled to see what info I could find on what types of questions are included in the different sections, and came up with this: http://www.prepstrong.com/pages/about-the-stanford-binet

It would seem to confirm my suspicions. Vocabulary can be influenced by reading together and how the parents speak, but it still comes down to exposure. We all modify our vocabulary choices when we speak to children, but we "dumb it down" to different degrees. And for procedural knowledge... well, that's just not going to be there if someone hasn't shown the child the procedures.

My unprofessional analysis is that, assuming an appropriate educational placement that can meet him at his natural pace, this child would shoot past most of his peers at crystallized knowledge in very short order, and you'll never see an FSIQ that low again.