Is it permissible to slaughter on behalf of one's son with the intention of warding off evil, using money mixed with a salary that has a suspicion of unlawfulness?

The questioner's statement: "I believe it is obligatory for me to sacrifice on his behalf" is invalid. Obligatory is only what Allah and His Messenger have made obligatory, and they have not made that obligatory. It is feared that this belief may stem from polytheistic beliefs. It is advised to strive for lawful earnings, as it is a condition for the acceptance of supplication, as stated in the Prophetic Hadeeth that the supplication of a man who eats and drinks from unlawful sources is not answered. Upon obtaining lawful earnings, one can give charity from it to ward off afflictions and diseases, while humbling oneself before Allah and asking Him for well-being, for in the Hadeeth: "Cure your sick with charity."