Managed migration from legacy benefits to UC query

I am hoping to get some advice if possible, please.

I have a son (aged 44) who is in receipt of Personal Independence Payment (enhanced rate for both components).
He also receives income related Employment Support Allowance. He mentions that he has a ‘joint’ claim for himself and his partner. For the ESA, he is in the support group.
His partner receives Carer’s Allowance.
They struggle because they have an over payment to pay back. They fell foul of the ‘working too many hours’ whilst in receipt of Carer’s allowance.

I have no sympathy for that.

My son rang me recently telling me he has had a letter stating that he must claim Universal Credit by March. I am guessing that this is the ‘migration’ that I have heard mentioned.
My son is dyslexic and struggles with letters, but he told a third party about the letter (by phone) who in turn told him that he’ll have no money for about 30 days because it happened to them (the 3rd party).
I haven’t seen the letter myself but intend to have a look at it next week.
Meanwhile, my wife is panicking because he has told her that he’ll have no food for a month.
Frankly, we will struggle to help him financially. His brother died in November 2024 (aged 46) suddenly and unexpectedly. He had diabetes since the age of 6 and had not been well.
He (the brother who died) had no life insurance nor money of any sort, and I had to scrape together the money for a funeral. I could do without having to fund the other son for a month.

Is anybody able to advise me as to the process of this ‘migration’ and is it correct that he will have no money at all for 30 days? It sounds a bit immoral to me, and perhaps even unlawful.

Certainly, I find it hard to think it might be correct.
How would he pay bills?

Thank you in advance for your constructive advice.