"Since we can't celebrate here on earth, I'm sending loads of birthday cheer to heaven" and "Thinking of you on your birthday, and missing you very much" are some birthday wishes for a deceased person. "Have a blast on your birthday in heaven" is another possible birthday wish.
When a loved one dies, it doesn't mean that his life still can't be celebrated on his birthday. Birthday wishes for a deceased loved one are an important way to show love and appreciation for him. He can't be physically touched, but a few special words can remind the people that knew him how much he affected the lives of his friends and family.