You will need
- - gift;
- - flowers;
- - postcard.
Tell me what for you is a great joy and honor to be invited to this day of birth, but, unfortunately, circumstances are such that to attend the event you can not. Do not limit yourself to vague references, be sure to provide details birthday, what case you got distracted, if that's not a secret.
Reassure the birthday boy that you are afflicted with the inability to be present at the bottom of the birth even more of it. As a "moral damages" will invite him out to lunch or dinner another day.
Prepare for the birthday gift and give it to him personally after the day of birth. To congratulate and give gifts in advance is not necessary: some are of the opinion that it's bad luck.
Order at the flower shop a beautiful bouquet of your favorite flowers on the birthday and pay for delivery by courier to the venue of the celebration. Flowers accompany a note or a post card, write your greetings and wishes and again about how you're sorry you were unable to attend on the occasion.
You will further temper the impression of his refusal to attend the celebration, if I don't forget to invite someone to your birthday when it comes.
To refuse the invitation to the birthday on the phone rude, find a way to do it in person.