I'm sorry to hear what you are going through. And it is totally unacceptable with the way your son is being treated.
I trained as a Teaching Assistand and if a child was suspected of having ADHD or any other special needs, whilst being assessed, I had to support them more in their learning. Once a child has been assessed and diagnosed, then the school should set a meeting in place. If you feel that your son needs that extra support, you are well in your rights to ask for it.
Some schools do not want to pay out extra money to have a teaching assistant giving one to one support nor do they want to use their current teaching assistants for this. Yet it is so wrong. This is how children end up being failed, when there are resources and help out there.
If it is going to help your son having an teaching assistant sit with him in a maths class and help with a subject, seperate from the class or vica versa, then he should get it.
I was a trainee and not getting paid, so the school I worked in took advantage of the fact but I saw some children being unsupported, which was such a shame.
Request a meeting with the head and Senco, take someone with you. before you have the meeting, write down the stuff you want to ask. If the school still insist that he cannot be offered addition support, request the school put this in writing.
I would also recommend you contact Advisory Centre for Education. It is a free number and they can give you proper, legal advice. they are fantastic and have helped me in the past.
Good luck and I Hope you get it sorted out.x