I have worked with a number of neuro-diverse children for whom Maths was their special subject.
For many of them, what they liked about Maths was the clear sense of right / wrong answers and they were often very quick at number bonds and times tables. These children tended to perform very well in arithmetic if it was presented the way they were used to but unfamiliar question presentation and reasoning tests were harder.
Here are some of the areas I have seen become a barrier as children have moved into upper primary. Sometimes these have caused meltdowns as children have found being asked to answer them uncomfortable or distressing or annoying. I think for some children, some of these feel like the teachers are breaking the rules of what they think Maths should be.
Do any of these seem like they might fit your son?
A) As they move through KS2, more ambiguity is introduced which some children really do not like.
e.g. 2 x 3 < < 4 x 4 (there are several possible answers)
B) Questions also get longer, with more exchanging, and harder to do in one's head which can be hard for children who don't like to write down their working out.
e.g. 500 - 297.
C) Questions are presented in a way which fit the page rather than make it easy to solve. e.g. written as 243 x 5 but to solve it, it is better to rewrite it as
243
x 5
Some children are very reluctant to rewrite out the question to make it easier to solve.
D) Increasing levels of multi-step questions which need careful reading
e.g. 24 children get on a bus. 2/3 get off. How many children are left?
(many give answers which are 2/3 of 24 which is how many got off, rather than how many are left. Or can give the answer of 1/3 when it needed to be how many children which is 1/3 of 24)
E) Lots of explain why questions which need a mixture of calculations and writing
which can be confusing / confronting e.g. Edward has £20 and spent 1/4. Paul had £16 and spent 1/2. Edward says he spent more money than Paul. Why is he wrong?
F) Questions where operations and scales of measure need careful attention.
e.g. Tommy's bottle held 3 L of water. Sarah's held 2 L of water. Tommy drinks 600 ml. Sarah drinks 400 ml. How many litres are left between them?
(Need to recognise what to add / what to subtract, convert measures, answer in the correct measures)
H) Interpreting bar charts / rulers / thermometers with different keys. e.g. if a key on a pictogram is that each is worth 5, then recognising that ** is 15 and not 3.
I do think it is possible for a child with a maths learning profile which finds all the above difficult to go from GDS to WTS over their KS2 schooling. For it to happen in a year seems quick, and for you to be taken by surprise also does not seem right.
But I hope the above gives some insight in areas where Maths abilities can get challenged.
This NCETM web page has assessments for each Y5 objective which might help. https://www.ncetm.org.uk/classroom-resources/cp-year-5-curriculum-map/