I am in Trafford (Manchester) and find the following workbooks very useful alongside using the relevant exercises in COAsT:
1. 3 x CGP workbooks: Non-verbal Reasoning, verbal-reasoning and maths ‘The 11+ Practice Book with Assessment Tests’ for the CEM Test
2. 3 x Letts 11+ Success for CEM workbooks ,Quick Practice Tests for 10 -11 years: Numeracy, non-verbal reasoning and English & verbal-reasoning.
3. CGP 10 minute tests: Comprehension, numeracy, non-verbal reasoning, verbal-reasoning (other subjects are also available)
4. A vocabulary workbook to extend vocabulary and practise synonyms & antonyms e.g. Synonyms and Antonyms by Christine Draper.
I have tried other publishers who have produced CEM practice workbooks and not found them as useful/exam-like as the above.
I would suggest that you start with the CGP workbooks (1) and then move on to the timed Letts workbooks (2) or timed CGP 10 minutes tests (3) in each subject… you probably won’t need both.
It would also be helpful for your son to have a go at a mock exam(s) (Trafford will be your nearest centre) – these will be advertised approx. February 2018.
Hope that helps.