Is there anything, non-school related, that he's interested in? I ask, because that's how my long-term home educated children learn. They don't have set pieces of work, or lessons, and at various times have had different interests, that they have researched, such as dinosaurs, botany, geology, art.
It sounds as if your son is a child that needs entertaining, needs a lot of attention. I would take the pressure off. Tell him you'll make a deal. He has a choice. He can try to do todays assignments, or his alternative is to go on the Internet and find out all he can about one subject. Tell him you will help him read any difficult words, but you want him to write down all he learns. If he doesn't know how to spell a word, not to worry, as long as he knows what he wrote when he reads it to you tonight. Give him a deadline of say, after tea tonight, tell him you will not nag him when he takes a break from it, it is his responsibility to get it done. Suggest subjects he would find fun, perhaps a hobby he already has or something if , given the chance, he would like to do. (Skate boarding? Travelling to Australia? sky diving?Martial arts?) He could find out everything he can about Lego
Get him to plan the days meals, Check whether you have ingredients for recipes he finds in books or online. Make some cakes together, with him reading out the instructions.
My son is very severely dyslexic. And at 12 had huge meltdowns. I understand how hard it is. Just keep reiterating to the school that you are trying to encourage him, but you are putting your child's current well being first. You will try to get him to complete assignments, but if they don't get done, sorry, you've done your best. Sometimes, when you take the pressure off, children cope better, and start to do things of their own volition. You won't be the only parent having these problems.