Migration to UC

Hello,
I am a carer for my 42yr son and have to help with form filling.
He had a managed migration letter to UC. Is this a voluntary migration and can he look for other benefits to see if he will be better off. We have not applied yet as still got time.
He gets income based esa and contributory esa, he is in the support group, and gets enhanced disability premium and severe disability premium, PIP and housing benefit at the moment which the letter says will stop at the date given. I have done a calculator check and although he gets the transitional element to make sure he is no worse off, this will only be for the first year then the transitional part depletes every time UC increases each year. . So his money remains the same every year until the transitional element gets to zero. Therefore it does not increase for a few years, so will be worse off than he is now in the long run.
Can he claim the new style esa instead of UC for his income. I know he has to get U C for his housing benefit.
And would he still get the enhanced disability and the severe disability with new the style esa.
I know this is lengthy, but just want to help him get the best possible outcome.