Is it too late now to change your mind and take the Yr4/5 option, if the school seemed appropriate in other ways? I hope the Irlens assessment helps a bit, though, as I guess you realise, it may not solve everything.
I often reply regarding younger children with Phonics, reading or writing difficulties. There is a Phonics Dictionary, which I think puts a lot of reading and spelling difficulties into an easy to use book suitable for children of all ages, though it is aimed at younger children.
There is a review of it in the MN book reviews section, so if you search Phonics and my name you should find it. It could go some way towards helping him improve his reading.
Regarding his writing, when I had boys who were VERY reluctant to write, I used to let them DICTATE their ideas to me, and I would type them on the computer. This relieves the child of the chore of handwriting, and encourages their ideas to flow more easily.
So if you have a tape recorder, or a phone he can use to record on, see if he can dictate a story, or a recount of some outing or special event. He can then listen back to it, make alterations if necessary, and at some other time, try to hand write it or type it. The important part is the ideas, descriptions etc, so even if you have to type it for him, it may encourage to do more in future.
I'll come back sometime to see the outcome of the Irlens tests.