Hi Galaxy,
Bit late putting this post in I think but just wanted to add my thoughts.
I think the letter is great. I wouldn't be afraid to show he worked in retail. The trick is to show perspective employers how this valuable experience can be applied in the classroom. He will have great experience of dealing with tricky customers on a face to face level as well as sourcing the correct product and suggesting alternatives. He will have good administration skills as well as excellant communication skills. All of these are positive things he can use to his benefit. Even things like cash handling at the till shows trust, organisational skills, and don't forget the most importent things, common sense and enthusiasum.
Good luck