May I suggest that the girls try something different with him other than shopping, i.e., a sports event or something different? I suspect he gets tired of taking them shopping and that's why he spends a lot of time w/your son, doing the "manly" thing of sitting around talking, etc.
I agree with snodderly about doing something different. Since your H previously accused of being used for his money, perhaps it's better to avoid going shopping with him. So he can't say he's being taken advantage of when the girls asks him to buy something for them. So he doesn't think they have an ulterior motive (of getting things from the store) in going shopping with them.
