Should it be "are" instead?

No.
Read the beginning of the sentence: Mr. Ng is one. Therefore, use the word is.

Mr Ng is one of our many patients who is receiving subsidised dialysis treatments and medications and other forms of care.

Mr Ng is one of our
many patients who are receiving subsidised dialysis treatments...
(Many patients are receiving subsidised dialysis treatments...Mr Ng is one of them.)

Does the above show that "are" should be used?

I commend Sterling for his sturgis efforts and should like to point out a salient snippet.

"Who is" is redundant.

Mr Ng is one of many patients receiving ....



Should it be
Could it be
Would it be
How good'll it be
And will there be a comma
Let it be

Should it be
Would it be
Could it be
How good'll it be
Whisper words of wisdom
Let it be