If he had been born after 1906, you would have written to Harrisburg, since all records after this date are stored there. But since he was born earlier, you have to go through the Bucks County Courthouse in Doylestown. You will also have to prove that you are a close relative of the deceased. The contact info for the courthouse may be found on the "Birth Certificates" page of the county's Web site:

http://www.buckscounty.org/government/rowOfficers/Prothonota...

Good luck!