I would have very firmly taken (not asked for) the present back, not least because with any luck the boy would have reacted badly to this and this would have hopefully prompted a conversation between the grandmother and her grandson about why you had done this.
What a shame for your grandson, minimo. It's so lovely to teach a young child the joys of giving as well as receiving. How confused and disappointed he must have felt, poor little lad 
What medical conditions seem more common now than when you were growing up?
Has anyone else noticed their energy levels change as they've got older?
What's one health habit you've started that's actually made a difference?

