For those people who are always able to choose to do something, this is a little glimpse into what it is like not to have that ability:
You are aware the task needs doing. You know you 'should' be able to do it, that it 'has' to be done. But as you approach it, it is like there is a sheet of glass between you and the task. You cannot reach it. You berate yourself. You still cannot get to it. You tell yourself that everyone else can do it, why can't you? You still can't reach it. You tell yourself that others are expecting you to do it. It doesn't help. The pressure pushes you further away from being able to do it, the sense of inadequacy, general crapness and incompetence pushes you further still, the self-loathing starts. I'm rubbish. I can't do anything. The task looms out of all proportion, growing larger and larger, distorting your day and sucking all enjoyment out of everything else. You still can't do it.
Restore perspective, reduce pressure, boost sense of competence
What else is your DS doing, apart from the school tasks? Ideally he would have time with you, time with friends and family, exercise, hobbies, time outside, sports, music, practical activities like cooking or making things, perhaps a group like Scouts - this helps put things into perspective so schoolwork doesn't dominate everything, it's just part of what happens, so it becomes less of a demand than if it is the main focus.
What is he good at, and how is this recognised and celebrated, both by himself and others?
Take the pressure off - turn 'demands' into 'options'. Here are some things which you could do: what would be most useful for you today? (This is REALLY useful for reluctant revisers, who get terrified/overwhelmed at a revision timetable - create a slot schedule and a separate list of tasks, choose tasks from the list as you get to the slots).
Rather than having a to-do list, have a ta-da! list: as you complete a task, write it on a list of accomplishments. This can provide a real boost and motivation to tackle more.
Getting started
Play 'task roulette' - I think I read about this on here. Write down a few tasks which need doing on index cards, pick one out at random, work on it for fifteen minutes.
If you don't want to do a whole task, don't! (Taking the pressure off...). Can you do 19 minutes? 12? 7? What is the smallest thing towards the task that you are currently able to do? Brilliant!
Schedule in time OFF, not time on. "At 11, we're going to..." That creates a natural time limit/end point for the task, which can make it less daunting.
Watch someone doing the thing on YouTube - there are lots of excellent maths videos. This shows your brain it is possible and primes you for doing the task.
Getting started and keeping going
Find a place which has enough stimulus to take the edge of the task. A silent, clear space can be inviting, or it can be almost unbearable because it becomes all about the task. A little noise or bustle can distract from this and allow you to get started. As Phineyj says, taking work out somewhere else can help, or have something undemanding on the radio or television in the background.
Body double - sit and work alongside, either on the same thing or a different thing. People chum up with others over the internet to do this.
Is there anyone like a friend or slightly younger neighbour/cousin he could work with or teach?