Ok that's really helpful and I agree he does sound fab, with an amazingly advavanced understanding of mathmatical concepts. Like you and others have mentioned his challenge with reading and writing need understanding because otherwise it will hold him back.
I would be pushing the school for a formal assessment, however you need to be aware of what this will and won't provide. A label of dyslexia provides a few benefits (like more time in tests and hopefully more support from school) but it won't actually solve his problem, for that you need to understand what his specific issues are.
Some of this you might be able to work out yourself. For example, can he blend a word if you sound it out for him? Let's take the word 'stand', some children, if the word is sounded out for them can 'hear' it straight away even though they can't do it if they sound it out themselves, others will struggle to 'hear' the beginning, others will struggle with the end sounds. This is all useful to know as you are pinpointing exactly what he struggles with.
Ultimately the solution is most likely that he just needs to spend more time doing it, with enough repition he'll get there, it will feel very different to him when maths comes as easily as breathing, however just because something is hard doesn't mean it isn't worthwhile.
I would ask the school for his phonics results - you are entitled to them. I would also be learning as much about phonics as possible. At this point you have a number of options - you could try teaching him a basic vocabulary using the whole word method via flash cards, focussing on the first 100 most used words first.
You could go back to basics with him phonetically, checking firstly his 1:1 coresspondances for the alphabet, then ones like sh, ch, th and then moving onto ai, ee, igh, etc. (search for the Letters and Sounds website) you could combine both, get the 100 most used words as flash cards and getting him to sound out say 5 until he is secure with them.
To do this so that it makes sense to him you need to understand the phonics code yourself perhaps explaining to him that written words are sound puzzles. For a lot of words the code is regular, using consistent rules. For other words the code is less regular (in the sense that the spellings are fairly rare). An example of this is the word bury. In most words ur will represent the /er/ sound (allowing for accents of course).
I found a good understanding of the phonics code really useful in supporting my DD learning to read, this is because although her school taught phonics the books they sent home were whole word books. I dealt with this by accelerating my DDs phonic knowledge so that she could understand how to sound out the phonetically tricky words herself, no guessing required. Knowing for example that if there is only one consonant between two vowels then you usually say the long vowel sound for the first vowel is a very helpful rule that applies some (but by no means all) of the time. It helps with understanding hopping versus hoping for example. Knowing that at the end of words y represents either /ee/ or /igh/. There are lots of rules to learn, but they do work for a considerable proportion of the words.
On Amazon you can get the Letters and Sounds work books and magnetic tiles with digraphs and trigraphs as well as the first 300 frequently used words. My DD had a few words on the fridge, she would sound the words out until she could read them without sounding them out, then we would move the words to the side and build a 'word wall', adding say 5 new words in a line to sound out and learn, add to the wall and so on. Soon enough she had a wall with 100s of little words, all learnt by sounding out (all be it that some were 'tricky').
There is a very cheap phonics programme called Phonics International, I think that the phonics code is free for you to download - it has all of the common and medium common correspondences. This programme has lots of written passages to work on specific correspondences and this may appeal to you as there are no pictures.
From Amazon you can order the relatively cheap Read Write Inc black and white booklets for their reading programme. The good thing about these is before you sit down to read a book you practice the phonics code, read or sound out the green (phonetically regular words) the red tricky words and after the story and comprehension questions you practice the speed words. I used to give one tick for correctly sounding out a word and two for reading it on sight (as in DD had sounded it out enough times to now know what it was).
I realise this is a lot of info for you, but I think your support will be key in helping your DS to succeed.
For your DS I would be explaining how important learning to read and write are and that he needs to spend say 10 minutes every morning learning basic phonics or sounding out words on the fridge (lots of praise, reenforcement of the rules) and 10 minutes reading from a non picture sheet or a Read Write Inc type book (i.e. a phonics book not a whole word book). Make this as much of a treat as you can, so sandwhich reading work between maths work, give him a biscuit / some sort of edible treat while he reads.
I would carry on getting him to read little bits of the books you read to him. Again I noticed on Amazon that there are quite a lot of maths children's story books, so high interest for him to help reinforce the idea that reading and maths are interlinked.
For writing get him a hand writing book with 4 lots of guide lines and practise spellings and little sentences with the spellings he's practising. No more than 10 minutes for each activity.
So half an hour in total for him, nearly every day but quite a bit more time for you depending on how good your knowledge is regarding phonics. The 'if at first you don't succeed try, try, try again' attitude will pay dividends. Of course I have no idea if you can find those 30 minutes easily, I just think that if you can it will pay dividends in the long run.