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 The Almighty to Cure your son from the sickness that has afflicted him, to Grant you the money you need to treat him and reward you amply.
Zakah is obligatory upon the money you collected to treat him since it reached the Nisaab and a lunar year elapsed on it. Zakah is a small amount that does not reduce the money; rather, it blesses and increases it.
Concerning the remuneration, no Zakah should be paid for it until a lunar year elapses on it. If a lunar year elapses on it and it is equal to or more than the Nisaab, you should pay Zakah for it. If a lunar year elapses on it and it reaches the Nisaab by adding it to another sum of money you possess on which a lunar year elapsed, you should pay Zakah for it as well.
Allaah Knows best.