"He is very unlikely to get or to need a statement if he is 9 and only 2 years behind. It's very unlikely he'll be the only one in the class that is 2 years behind."
That is simply not true. None of us here can tell the OP whether her DS needs a Statement. That is the role of the LA to decide. The SEN Code of Practice sets the threshold as 'provision required is beyond the resources of the school from within it's own budget'. Equally, the only way to decide the provision needed is to look at progress made. The threshold for intervention is 'inadequate progress despite appropriate intervention from school'.
Also, we have no idea what the OP's DS has been learning to date in Spain, and may already be significantly behind UK peers even if he is 'only' 2 years behind in Spain.
My DD is 'only' 1½ years behind in Maths - she's on a 1c now in Year 2. She has a Statement and goes to Special School.
HelenAnneArt, I would contact the LA you are moving to. I would initially be cautious about mentioning 'dyslexia' because it is rarely taken too seriously in the UK. I would focus on 'undefined learning difficulties'.
You can go to the GP and ask for a referral to a developmental paediatrician. Have a think as to whether you have concerns in the following areas:
-Gross motor skills (running, skipping, hopping, riding a bike, etc.)
-Fine motor skills (writing, drawing, zips, buttons, knobs, etc.)
-Speech and language (both expressive language and speech sounds, plus understanding instructions verbally)
-reading
-academic skills
-medical features
-self-care skills (so dressing, washing, etc.)
-danger awareness/independence skills (out and about, supervision needed etc)
-social skills (including friendships, turn taking, rule adherence, etc).
Come over and speak to us on Special Needs - we have all the information and can share which Counties to avoid how things work in different areas.