It's always tricky to give definitive answers without knowing all the facts.

Based solely on what you've described I see huge red flags saying scam.

Your son is not the first and will not be the last.

The difficulty now is that he has strong emotional ties and may find it difficult to form objective views.

Whatever the truth he must not send any more money. Period.

Previous advice regarding direct contact to Doctors, hospitals etc is crucial but he must be sure the contact numbers and details are genuine.

I'm very sorry to learn about this.
It's always sad whenever such things happen.

He should just stop sending money and clearly state he does not have any more to send

The resulting discussion between them will gradually reveal clues as to what's going on.

All you have described as happening within that short time is not normal at all.

It would be a costly mistake both financially and emotionally to continue to the point of marriage and UK visa.

We do have members here who have personal experience.
Why not have you son join our community and hopefully be guided both those who have travelled the same path?