"My, my, isn't if it isn't my special young man's birthday? Gosh, you've grown up so fast. Of course, with mommy feeding you loads of cum year after year I knew you'd grow up big and strong. Speaking of which, I made sure to dump plenty of loads into your cake batter, just like you wanted. I guess I'm feeling extra horny today, because I'm pretty sure the cake is more jizz than batter at this point. But that's baking in the oven right now, and its all for you on your special day."
At which point you look down and notice the "present"
"Oh, this? Yes, of course mommy got you a present on your big day."
This is when you can kind of start turning and "modeling" the birthday gift in profile.
"Oh where are my manners? I bet you want to just rip it open and enjoy what mommy gave you. But you know how I love to tease you. Can you guess what it is?"
Then here you pretend that I guessed wrong.
"Nope, that isn't it."
Then another guess
"Not that either."
You continue to model and show off the package.
"How about I give you some hints? It's big. It's hard. You love it in your mouth and ass. And it's growing inside this box as we speak."
At which point you stop modeling it.
"Maybe we should wait until after you've had your birthday cake. After all, don't we always have cake and ice cream before we open presents."
At which point you can start modeling it again.
"I mean, I don't mind carrying around this present a little while longer. I carry it around every day, after all.
...
Oh, all right. Go ahead and open it up." You take the present off revealing your cock.
"That's right, mommy's got a big cock for you to suck for your birthday. I knew that even though you want all that cum in your cake, you need just one more big load right in your stomach." Cum suck mommy's cock like a good boy, I know you love sucking up to me.
Now it's time to sing you "Happy Birthday" and it's going to be a special cum countdown verse.
"Happy Birthday... 10... to you... 9...
Happy birthday... 8... to you... 7...
Happy... 6... birthday... 5... dear cocksucker... 4...
Happy birthday... 3... to... 2... you... 1...
Now blow my big birthday candle..."
"Now don't you worry son, this gift is built to last. It's getting hard again already. I'm sure you'll want to use this gift on your ass soon enough, but sit down and enjoy your cake right now it's full of delicious tasty spunk don't want it to go cold now do we. You're gonna have 'loads' of fun with your new gift."