So call up DS and tell him that you're disappointed that he didn't even have a CARD for his Dad's birthday. What's he going to do? Not call you? Be unpleasant at a family dinner? Huh? Feel free to tell him that too!
Can you imagine? "I'm not afraid of you. You're acting like a brat and as your mother, I will ALWAYS call you on it. What are you going to do? Call us LESS than never? Ha!"
Sorry they're being stinky Pen, you deserve better.