The best birthday message for a business associate is one that addresses him personally and wishes him the best for the upcoming year, such as "All of us at our company wish you a happy birthday and another year of success." The message should be personal but remain professional.
The message should show that the company giving the birthday message values the business associate and has his interests in mind. Another example would be "We value your business and look forward to another year of working with you. Happy birthday from our company." A handwritten note inside a card is an added personal touch that many businesses use.