The National Curriculum provided by mrs is great but considering that you have just 4 weeks left. You need to get practice tests for SPAG which appears to be the area of concern you have.
All the major UK SATs publishers are tailored to match the 2014 National Curriculum requirements so its best to dive into using any of the major publishers considering the 4 weeks constraints.
Of all the major publishers, CGP seems the most popular and I actually found them useful for SPAG, Reading and Maths.
You can get practice materials directly from the publishers : (CGP, Letts, Scholastic, Schofield & Sims). However, getting them on Amazon may just be faster for you as you can get them on next day basis. I have tried DD using the four publisher practice questions and taken her through the revision guides she's pretty comfortable in terms of results and time pacing/management having identified/exposed her weak areas using all the tests. We're know concentrating on areas requiring improvement.
I will recommend the KS1/KS2 Schofield & Sims Revision Guide / Practice Tests for Maths & English as they perfectly cover the requirements of the National Curriculum and are the toughest of all the published work I have seen. You need not worry about adjoining years - 1,3,4& 5 where SATs are not taken. Take your son through the Revision Guides and get practice materials. That'll be my advise considering your time constraints.
The List is recommended for KS2.
English Practice Papers & Revision Guide
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Key Stage 2 English Practice Papers: KS2 English, Ages 7-11 (SATs for new national curriculum) (Schofield & Sims Practice Papers)
Key Stage 2 English Revision Guide: KS2 English, Ages 7-11 (SATs for new national curriculum) (Schofield & Sims Revision Guides)
Maths Practice Papers & Revision Guide
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Key Stage 2 Maths Practice Papers: KS2 Maths, Ages 7-11 (SATs for new national curriculum) (Schofield & Sims Practice Papers)
Key Stage 2 Maths Revision Guide: KS2 Maths, Ages 7-11 (SATs for new national curriculum) (Schofield & Sims Revision Guides)
Science Practice Papers & Revision Guide
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Key Stage 2 Science Practice Papers: KS2 Science, Ages 7-11 (SATs for new national curriculum) (Schofield & Sims Practice Papers)
Key Stage 2 Science Revision Guide: KS2 Science, Ages 7-11 (SATs for new national curriculum) (Schofield & Sims Revision Guides)
The major publishers have done the work of mapping the national curriculum requirements and they've done a great job there. Lest I forget, buy books published in the years 2016 and 2017 to be on the safe side.
Best of Luck