Has anyone had an issue with splitting a joint ESA claim?

My son has claimed ESA all his life, since he could. He met and married a women who also had her own claim. Both are 30years old. Both got Full supported ESA both get enhanced PIP in both areas. When they married, they were told it had to be one joint ESA claim. He joined her claim, 4 years ago. In August they separated.

They were told they had to change benefits to UC. Two single claims. She now gets Full Rent, standard UC, the support Payment and a further transitional sum. she also got the government cost of living support payment. There was no break in moving from ESA to UC. No work capability assessment.

My son was told, even though he was forced to be a joint ESA claim four years ago, that because he was on her claim, he is not protected. He gets, rent, standard UC, no transitional payment, no cost of living payment from the Government. He had to wait 3 months before getting the support payment. He had to go through a capability for work assessment. This is fundamentally unfair, and is prejudicial to my son. Has anyone been able to successfully challenge this?