I suggest you take him to puppy training classes, preferably clicker training, as this is a gentle and fun way for both you and your dog to learn. Your local vet will know where the nearest classes are held. If not, phone your nearest RSPCA and they might know. You pup will soon learn in a fun way that the barking is not acceptable to you. Please don't put this off, it is important you do this when he is still a puppy as it is much easier at this age. Best of luck!
Info on clicker training can be found at:
http://www.clickerzoneuk.co.uk/sking2.html and
here:http://www.clickertraining.com/training/clicker_basics/ It is a US site which also has good info.