Oh, I don't know, but I have to ask. My grandfather's nickname is Poppie! Is your username a nickname, too, or is my pronunciation off? Never met another, you'd be the first.
You can mention to your checkin CM that you are celebrating his birthday. Sometimes they will send a surprise to your room. Mention it when you make your room reservations or dinner reservations. Or, plan a party at a restaurant. I have no experience with this. I hear Chef Mickey's is great for birthday dinners, with characters. They have a decorate your own cupcake with Goofy station for dessert.
Or get him the Kitchen Sink ice cream sundae at Beaches and Cream (Beach Club). Its gargantuan and you'll never finish it. 1 scoop of every flavor and every topping.