Why would he say this? Is it a common expression?
No, this is not a common expression. However, he says this here because it is common for people to have a "wish" on their birthday. Usually this is when someone gets a cake and the cake has candles on it. Before blowing out the candles, it is tradition to "make a wish". So he says that he is going to give him his wishes here since it is his birthday and he gets a wish.
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