The very best grocery bags to have are what was used in EU in the 80's...a crocheted, mesh bag. It folds up to nothing so he can cram it in his pocket and it expands to hold TONS without breaking or tearing. Here is a free pattern I looked up for you along with a picture of what I'm talking about. I have all kinds of bags in the back of my car and this remains my favorite.
http://theadventuresofcassie.blogspo...ocery-bag.html