He needs to somehow file for emergency custody for court.
Be sure that he documents everything. Unfortunately until a court says otherwise both parents have equal rights. She should not be hiding the kids from him but there is nothing preventing her from doing this.
He should do whatever it takes to be able to hire an attorney.
Who has told them he will be responsible for all court costs? That is why he needs to get some legal representation.
In the future, please post questions like this in the Family Law part of this site.
What state is he in? He should go over to his courts law library/self help dept and research what he can do.
I hate to say this but do the kids go to school? Can we see them there? He needs to start getting tough with her and demand to see his children.