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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
We ask Allaah to grant us, you and your child health and cure. We would like to tell you that seeking the help of Allaah through performing prayer and supplicating to Him is legislated and one of the best means of fulfilling needs, especially if this is done in the last part of the night or after the obligatory prayers. The Prophet, sallallaahu ‘alayhi wa sallam, was asked: 'What supplication is most likely to be heard (and responded to)?' He, sallallaahu ‘alayhi wa sallam, said: “(That which is offered) in the last part of the night and at the end of the prescribed prayers."[At-Tirmithi]
For more benefit, please refer to Fataawa 83421and 85023.
You have to ask Allaah, The Almighty, to cure the child and to guide doctors to the proper way of treating him. Out of the mercy of Allaah is that He did not send down any disease except that He also sent down the cure for it, as the Prophet, sallallaahu ‘alayhi wa sallam, said: "O slaves of Allaah, seek cure for sickness, for Allaah does not create any disease but He also creates the cure for it except for one disease." They said, “O Messenger of Allaah, what is that?” He said, "Old age." [Abu Daawood, An-Nasaa'i, At-Tirmithi and Ibn Maajah]
Moreover, you should adhere to saying "Hasbiya Allaah laa ilaaha illa huwa ‘alayhi tawakkaltu wa huwa rabbu al-‘Arsh al-‘atheem" (Sufficient for me is Allaah, besides Whom there is no other god; in Him I put my trust and He is the Lord of the mighty Throne) seven times in the morning and seven times in the evening for the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever says every day, in the morning and the evening, Hasbiya Allaah laa ilaaha illa huwa ‘alayhi tawakkaltu wa huwa rabbu al-‘Arsh il-‘atheem, seven times, Allaah will suffice him with regard to what he is worried about from the matters of this world and the Hereafter.” [Ibn As-Sunni - Al-Arnaa’oot graded it Saheeh (sound)]
You should drink much Zamzam water and let the child drink from it and supplicate Allaah, The Almighty, for him while drinking. It is better to read Al-Faatihah on the water for there is a great benefit in that as confirmed by experience. Ibn Al-Qayyim stated in Zaad Al-Ma'aad: "There was a time when I fell sick in Makkah, and I could not find a doctor or any medicine, so I treated myself by reciting it (meaning Al-Faatihah). I would take some Zamzam water and recite it over it several times, then drink it. I recovered fully by these means, then I began to do that in many incidents of pain, and I benefited greatly from it."
You should give a great deal of charity to the poor and feed fasting people so that their prayers may benefit you, by the permission of Allaah, The Almighty. Al-Bayhaqi stated in Shu‘ab Al-Eemaan that a man asked ‘Abdullaah ibn Al-Mubaarak saying: "O Abu ‘Abdur-Rahmaan, I have an ulcer that had sprung out on my knee seven years ago, and I tried to treat it with various treatments and I asked doctors but to no avail." So Ibn Al-Mubaarak said: "Go and dig a well in a place where people need water, indeed I hope that water springs up there and your bleeding stops." The man did that and Allaah, The Almighty, healed him.
Allaah Knows best.