This would really annoy and upset me too Alexa
This question of thanking someone for a present applies to EVERYONE, young and old.
If I have taken the time and trouble to buy someone (friend or family) a present, or had given them money as a gift, it should be acknowledged by a 'thank you'.
I am not interested in any excuses about 'oh it's how things are these days with he young'. To not thank you is incredibly rude isn't it?
In your position I would make my feelings known to the young man, and if nothing changed then the gifts would stop, grandson or not.
Do you think you know when you are going to die?


