I think you might need to seek expert advice on this question as there are so many aspects to it. I’m guessing it depends on how an expert assesses his capacity, and then whether you are able to apply for LPA/Court of Protection, whether anyone will actually give you a mortgage, what the implications are for housing benefit/his care contribution/any future care you personally might need if you are living with him etc. If you are at a point where you are considering future care and accommodation for your son when you are no longer around, perhaps contact adult social care and ask about his options for living in his own own home or supported living with others and so on.
From what I know a regular secure income is what they are looking for. They like £20/25k per annum. Enough to cover the mortgage with stress test of 145% incase of any future increases. Talk to a good mortgage broker, all of market. You will need a deposit too. 15-25% of purchase price. Local authority do schemes too. Part buy part rent. Worth a look. Good luck.
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