Edexcel RS Syllabus modules 4 and 11.
Doing both modules is a full RS GCSE. Both modules are the Islamic Studies modules.
Student books for each are available from Amazon - about £12 each.
I also ordered the £72 teacher's book for module 4 but it was pretty useless to be frank so I actually sent it back to Amazon for a refund.
I have bought Examining Religions - Islam by Ruqaiyyah Waris Maqsood as a supplementary book.
It is all pretty straightforward. I did RE A level (Christianity/Islam) so have some understanding of the skills/exam answers that are needed.
Ds1's school have been supportive, saying they will book him for the exams (but only if he gets an A or A*
!) and the RE teacher has said she will give me some materials too.
I wouldn't say the standard is poor. Certainly, a good standard of English and a mature level of thinking, is required.
I like the way it encourages students to look at Islam with a 'contemporary' eye and link its teachings to the modern world that we live in.