RBBMummy, I suggest you have a look at the G&T board here, especially threads on '2E', children who are gifted and have SEN at the same time.
Because you really need to recognise (for your DS's sake) that your DS's academic abilities are exceptional. At least regarding reading (on the side, is he hyperlexic?) and his knowledge of how things work, you do not mention maths much. Gifted children can have a lot of problems at school. Their gifted-ness is often not really catered for. At the same time children with SEN are also often not catered for very well. Some schools are better at these things than others but you can't count on it. Children with dual exceptionality (2E) often suffer at school.
A friend's DD is highly able, academically (read fluently, and taught herself all TimesTables including understanding them before starting school, photographic memory -> these things are exceptional), but has serious behavioural problems (no diagnosis). Her first year at school was ALL about behaviour. All her 'learning objectives' revolved around behaviour. That year was very very challenging for her, and she learned a lot, but not academically, at all. IMO (and in her parents' opinion) it was fantastic that school focused on, and helped her to progress, in the areas she was struggling, rather than 'resting' on her academic abilities.
But it also meant she was not intellectually challenged at all during that year. If this is the thing your DS 'obsesses' about (puzzles and working things out) then it will take a very careful, thought through approach by the school to use his interests, not to extend his academic abilities, but to work on his social skills/any autism related difficulties.
Also if you read the EYFS framework as has been suggested on this thread, you'll see that the 'prime' areas of learning in reception are
• communication and language
• physical development
• personal, social and emotional development
and if your DS struggles with any of this, most settings would focus on that before worrying at all about how his abilities in reading or understanding the world, where he meets all end-of-year requirements already, can be stretched.