My DS 7 is in Y2, has a recent diagnosis of moderate dyslexia and is 2 years behind. He has just been issued with a statement.
I have a daughter who is Aspergers with high academic ability and she also has a statement!
It is hard to know without knowing all the ins and outs whether a child needs a statement or not.
My sons statement is not purely for his dyslexia as he also has social anxiety, auditory processing difficulties, fine motor skill difficulties and some speech and language difficulties.
With your son having autism and ADHD he will also probably have needs beyond his dyslexia.
School are expected to fund the first £6000 of SEN provision for a child. They receive a SEN notional budget for this and if their budget is not enough to cover what your son needs then they can apply to the LA for top up funding. Funding is not your issue. They need to meet his needs regardless. To not meet his ASD/dyslexic needs is disability discrimination.
A school doesn't have to use an IEP but they have to use something else instead. If school are failing to meet his needs then you should follow the school's complaints procedure. You can make a claim for disability discrimination if they continue to refuse to meet his needs.
Sorry if doesn't read very well as on my phone.