Is it permissible for an Imam to receive a salary from the Ministry of Endowments for leading prayers in a mosque he built at his own expense, and in which he only prays during holidays and special occasions, knowing that he covers the mosque's expenses, such as electricity and cleaning, and what should he do with the money he previously received if the salary is impermissible?

This money is a reward for your work as an Imam in the mosque. Since you did not perform the work due, it is not permissible for you or your son to take it. This is considered consuming people's wealth unlawfully, for the wealth of Muslims is sacred. You both must return what you took of this money to the responsible party. It is not permissible for you to deduct water and electricity bills from it, because you paid them as a donation, and it is not permissible for you to revoke your gift.