Have a meeting with them, take a letter with you stating that they are failing to safeguard your brother for reasons x, y and z. That this means there are consequences a, b and c. This means that they are failing your brother. In order to properly safeguard your brother, they will need to address x, y and z, and that you propose the best way to do this is p,q,r.
Point out that when your db was told he was going to be moved to this class, he predicted that he would suffer the consequences listed above, was told to give it a go which he did, but has not been supported despite suffering the consequences. The previous meeting with the school showed a callous and dismissive attitude to your brother and the negative effects that their actions have had on his life, because you think that they could not be bothered to deal with the issues. and in so doing they are failing in their duty of safeguarding your brother.
You would like to discuss this with them in the meeting to find out exactly what they are going to do to rectify this failure and what actions they are going to put into place to ensure that your brother is properly safeguarded at school.
Take notes - or record the meeting on your phone - you can, as long as it's not a hidden recording - just breezily get your phone out (get a voice recording app so it doesn't take up loads of storage!) and say that you're sure they don't mind if you record the meeting so that you both have an accurate record of the meeting and that you assume that they will be doing the same (and if they say no, ask why, that it's very suspicious behaviour, suggests that they have something to hide and that if you both have your own recordings then what can be the harm in it?)
if there is a senco then get them involved too, and make sure that your letter points out how they are discriminating against his disability which isn't allowed (well I think, not sure of the exact wording of the law!)
Good luck