As someone who’s finished GCSEs, I went to a school where some students were international so took certain GCSEs (mainly languages) early but I haven’t seen anyone take science early so didn’t know it to be possible. Given your brother’s situation of being a private candidate I believe this would be possible, but you would have to talk to the examination centre. Also it’s better in general to take gcse science over both years due to the high content, I’d say it would be easier to take them all in the same year.