I can pretty much guarantee you would have similar problems with lack of accuracy at m/s. The scores do sound low given your DS is same age as mine but yours is much further ahead with language eg ds is still on basic wh questions (and has been for 2 years) and still only asks wh questions very occasionally. But we have the opposite problem DS scores are too high (1c literacy - which is a total joke) and 1a maths (which may be right but conceals huge gaps with vocab and using numbers rather than just rote learning patterns). Teachers just don't get him at all - fail to realise how much is just a rote learnt string of information which he does not understand. We are still waiting (in 3rd year) for an academic curriculum which is even vaguely fitted to DS actual needs - Doomsday post resonates with me - we have to prove he can learn by teaching it at home for them to sit up and take notice - the work is either too easy or too hard so he switches off, and I am at the point of just accepting it is never going to happen - the m/s teacher is timetabled to the max and has no time for DS who needs work differentiated well below the level of the others in literacy, science etc. It would be quicker and easier for me to plan for him myself and send in my own worksheets for him but the HT will still be insisting the teacher is doing all the planning necessary and probably refuse to let me.
For us the ipad is our friend. DS has some of the montessori maths apps which he loves and 'maths builder - primary' by Paul Fowler - someone on here recommended it. It covers the maths KS1 curriculum. He went from P4 to 1c in about 4 weeks using this and then took his love of maths to youtube and BBC games etc and is now 1a. All this has been achieved by DS himself with zero input from a qualified teacher.
I am slightly envious about the way your school 'gets' the real life / functional / meaningful side - although I can see why you see this as low expectations. Much of what the teacher wants us to do with DS is not meaningful or functional at all, its just rote. At the moment she is anxious if he doesn't grasp tens and units fully he won't be able to do partitioning which is the (only) way they do addition for larger numbers. Now first I suspect DS can do tens and units and probably can do partitioning, but so what if he can't. Frankly when you have a brain that remembers lots of sums in your head and are likely to have a calculator on the phone in your pocket in future life what does it matter for DS if he can't add up using partitioning. I have A level maths and I never did partitioning. This is where mainstream goes Gove / OFSTED crazy and loses all perspective that there is more than one way to teach a skill and some skills are pointless for some children anyway. I wish DS teachers would grasp that going to the shop and using money is more important for DS than partitioning large numbers. All they can see is whats next on the curriculum and they move on even if 50% of the class hasn't got it.
I would also say that getting past NC1 is hard - more about abstract concepts, inference etc etc so actually by filling those gaps your DS will be better able to move up to level 2/3 whereas in mainstream those gaps would just be left. DS is increasingly being excluded from lessons because of the language barrier. The CT is just beginning to realise how big a barrier that is but goodness we are well into year 2 and the penny is only just dropping and even then we don't have any actual work set by the teacher to address this.
I think the choice is move to m/s and accept you will have to teach language, inference etc etc and make the work meaningful or stay where you are and do the academics at home. There are more ready made resources for the academics so I think it might be a case of the grass being greener...