Your problem, Eva, is that he is no longer a child, he is an adult, and therefore a Child Settlement visa is out of the question.
Also, based upon what you have posted, he is not "living alone outside the United Kingdom in the most exceptional compassionate circumstances'', and therefore I really can't see a path for him to move to the UK.
I shall be very pleased is someone else can think of a way to achieve what you want to do.