I haven't read this whole thread but my thought about your DS is that he doesn't want to fail. So with reading, for instance, he does not want to risk trying and getting words wrong,so he's just not going to do it at all. DS was similar at that age. When I took him in for an IQ test (in his case we were investigating developmental delays), I was able to watch the test, and DS was randomly pointing at answers while smirking. He acted up and got silly/goofy. He was the same way at home whenever we asked him anything that resembled an academic question. Even if it was something simple that he should have no problem with, he shut down and refused to engage.
One thing that you could try with reading is to get him some audiobooks with the paper books to follow along, and just leave him alone with them. That's how DD learned to read. And of course, keep doing what you're doing and read to him as much as you want. When he is ready, he will probably just "take off" and read fluently with seemingly no effort.
DS has developmental coordination disorder and was able to walk up and down steps with alternating feet at age 3. This may be another thing where he is scared he might fail and doesn't want to try until he's sure of himself. At age 3 DS still scored in the average range for gross and fine motor but I knew something wasn't right because he was so deficient in terms of a few specific skills, like holding a pencil. He was able to write his name, albeit messily so he "passed" in terms of 3 year old skills, but then simply did not make much progress after that and rapidly fell down the percentile charts.
He would learn how to do things like skip (I think he was almost 5), but then forget. Now 3 years later, he still looks really awkward skipping. He looked really awkward doing a lot of things, but this "awkwardness" doesn't show up on standardized tests. The standardized tests for motor skills are not perfect, but you should probably get a motor skills assessment (for both fine and gross motor) and then get another one in about a year and make sure there is progress. The school did the Test of Gross Motor Development (or something like that) and the OT/PT always did the BOT2. The test that the school did was better at showing his issues.
