All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The sign in a form of a crescent at the back of the head of your uncle's son could be a normal matter that has no effect, upon which he is created and it could be a disease which will has its effects like pain and the like. It is only the specialist doctors who may determine this. If it is found that it is a disease, then you may assign an appropriate treatment for it, and there is no disease except that there is a cure for it, as stated by the Prophet .
There is no harm also to perform Ruqyah by reading some Quranic verses, Prophetic supplications and other legislated mention of Allaah. Indeed this is desirable even if one is not sick, as a way of protecting the children. The Prophet used to make Ruqyah to Al-Hassan and Al-Husseyn saying: "Your father (i.e. Ibraaheem) used to perform this Ruqyah on Ismaa'eel and Is-Haaq by saying: "O'eethukumaa bikalimaatil-laahit-taammah, min kulli shaytaanin wahaammah, wamin kulli 'aynin laammah. "I commend you two to the protection of Allaah’s perfect words from every devil, vermin, and every evil eye." [Al-Bukhari]
Finally, it should be noted that one should not resort to magicians and fortune-tellers. It is confirmed that the Prophet said: "Whoever goes to a fortune-teller and asks him (about something) and believes his sayings, he has indeed disbelieved in what has been revealed to Muhammad (i.e. disbelieved in the Message of Islam)." [Abu Daawood]
Allaah Knows best.