I'm currently trying to prove the case for Home ed as opposed to a school. (Have put in formal request to change part 4 of statement) so can see where you are coming from. In my case using Briteschool for years 5 & 6 is doing the job for me at a cost of £100 a month and 6 hours online tuition a week. I'd never have proved DS can work at year 8 level for science in year 5 on my own.
Maths
For proving my point re maths, I used the free online assessment that Mathswhizz offers (make sure you use the UK version!). It gives a maths age for each of the NC topic areas. I then repeated this annually to show progress (or lack of last year!). Being able to show that DS has gone from age 4.5 to age 7.2 in 3 months last term in his weakest area kinda proved a point to the Ed Pysche 
who was so reassured they put off a home visit (not quite what I intended but hey ho!). this year i've paid for a subscription, but please don't feel obliged to. Just doing the free assessment every 6 months or so will make your point as it's all NC mapped.
We follow the free resources offered by plymouth Uni, I adapt the lesson plans to a more numicon style to meet DS's special needs. This means currently we are a year ahead for graphs and visual stuff and working from the Year 2/3 lesson plans in his weak area. www.cimt.plymouth.ac.uk/
The teaching methodology is based on the Hungarian style, which is far more logical and sensible than the standard ology used in state schools, but it's all pre-mapped fro you against the National Curriculum which takes away a whole heap of stress when trying to satisfy the carrots and cater to a very spikey profile! I really recommend you take a look at it. There are 175 properly planned (you'll have to do your own differention!) for each school year from reception right through to GCSE.
Science
For science following Charlotte Mason (ambleside online) with it's heavy emphasis on nature studies got DS to a level 2a at the end of Year 2. The ONLY subject where his NC score matched his IQ. I'd rec that if your son, is as I think still in Key Stage 1.
Literacy
For literacy I'll freely admit I've had to deviate from the official NC, as DS just has too many ishoos. I will be sticking with headsprout as it's doing what it says on the tin. I don't care how the hell they mark it so long as DS manages to learn to read and write, neither I think do the LA as so many methods were tried and failed by his schools over the years. I think they have the same attitude as I do, so long as I spend a couple of hours a day TRYING to get DS up to speed they'll see it as a result.
This resource I am reliably told maps to year 3 & DS enjoyed doing it. He also got a lot out of it in terms of his ASD ishoos. www.amazon.co.uk/Literature-Pockets-Evan-Moor-Educational-Publishers/dp/1557998744/ref=sr_1_fkmr1_1?ie=UTF8&qid=1388588356&sr=8-1-fkmr1&keywords=literacy+pocket+aesops+fables
It may be worth having a look at the same publisher, as it gives you evidence of structure to your teaching if nothing else.
The Ed Pysche has agreed to formally test for me as DS has so many issues that impact his literacy that it's not even funny. This is the area where both the LA & I have agreed that we'll be flexible & that they don't call it "special needs" for nothing lol!
Whole child stuff (so you don't get accused of offering to narrow a curriculum)
For the arts they mainly expect "exposure" to various art forms. A trip to a gallery plus a drawing, painting or model is enough. Or a child that can tell you about a play he's seen etc. Sports it's the same. If they enjoy art or learning an instrument you are onto a winner!
PSHE is life skills for special needs kids rather than too much sex ed at this age, anything you do to help his social and communications can come under this banner as well. I feed a lot of the social skills & language stuff as evidence for literacy too. All the language for thinking etc fits here and under literacy without any grief.
IT - let your child produce some written work on the computer from time to time and you can tick this box with no fretting. Just show em the typed poem, or picture or whatever.
socialisation
Due to your son's diagnosis - They will expect evidence of socialisation, preferably something that can be independently verified for attendance. Sunday school, scouts, weekly homeschool group, swimming lessons count. Play dates don't with general carrots like the EWO (which is really unfair when dealing with a kid with ASD imho!).
I got this advice from someone who is running an Aba style home programme for her 11 year old fully funded. This is the bit she's seen other parents lose their fight over, & find themselves forced back into SS -the lack of what the LA regard as socialisation. She's a bit of an expert and has advised other parents successfully on how to get funded home programmes so I'm listening to her advice iykwim, as it seems a silly hurdle to fall on. however by now you are well acquainted with LA predjudices. being in a new area this is the bit that's gonna take the longest to get fully resolved as I'm still finding out what's on offer as our kids can't just join in any old activity iykwim.