My Brother's Ex is being difficult over childcare arrangements - she never lets him know when their boy (12)?

will be coming to his or not - yesterday he came after school because she was working but he was not in - their 18 year old was but that was just by chance. He has tried e-mailing her and texting asking her just to let him know what is happening and she never replies. What can he do? He adores his kids and she undermines his ability to co-parent by never letting him or indeed the kids know what is happening. Thanks - we are in the UK.