Have a couple friends walk past your door about every minute (one by one then together then together and talking) while you sit on the floor close to the door with VERY yummy and TINY size treats. Each time he hears them and before he starts barking, say good boy (in a very sweet tone, you know that protocal
and give him a treat. If he gets a bark out first, then you say ACK (a bit loud) and touch him at the same time to re-direct his attention, then when he quits, praise and give a treat. Keep this up for about 15 to 20 minutes three times a day. While he is in training, keep him as far away from that door as possible, even if you have to crate him in the bedroom and just take him outside every hour or so. If you seriously work at this, it shouldn't take more than two or three days.