The UK has the Mental Capacity Act. It's important to recognise tjat lack of capacity is limited - a person may not have capacity to look after their own money but you should not assume that they lack capacity to decide where to live for example.
Informed consent is mostly not that informed. Clinicians don't understand risk, cannot communicate risk to patients, and patients can't understamd that communication.
The experience of the decades of life he could remember? I don't see how postmortem brain scanning is any worse than doing memory tests on him when he's alive.
And children can agree to things just fine through their guardians.
He was 20-something when he lost his memory. So barely an adult.
And memory tests are not quite the same as dissecting his brain on television, then putting it on permanent display in grotesque life-sized cross-sections.
Further, he was institutionalized. So the guardians were - who? the doctors doing the dissecting?
You say "grotesque" in several of your comments, but this is a value judgment that is not shared by all. After death, what could be more poetic than using the instrument of one's intellect, the brain, to uplift the intellects of others?
And you, as an educated 21st-century world citizen can make judgements like that for yourself. This guy arguably can't. That's the whole issue, ignoring semantic arguments about word choice.
