Sorry for the delayed response but thanks to all for your suggestions.
Bryony- fabulous suggestion I’ve added that to my ‘to buy’ list.
Sara- he will only loosely use an occasional Makaton sign with me on a 1:1 basis, and only in imitation of my signs. He has rejected the idea of signing as a regular go to tool for himself. However, I did consider use of their symbols as they are quite systematic in their design.
Lots to consider as he is more used to widgit symbols having been exposed to these in various forms around school.
Susannah- thank you, I will look up that study- I now remember that I read the Aphasia therapy file when at uni written by Byng, Duchan and Pound that had a chapter- I found reference to it but sadly no longer have the book- C. Sacchett, J. Lindsay, Revealing Competence and Rethinking Identity in Severe Aphasia using Drawing and a Communication Book.
I feel that I am on the right lines so thanks again!