There is a form of training called NILF ~ which is a very gentle way of reinforcing who is leader in the house. It has certainly worked with our dominant Westie.
NILF Training
However, this sounds like more of a fear response than simply attacking. It happens only when the doorbell rings or someone knocks at the door?
First of all, he should not be allowed anywhere near the door area. That space is YOURS and you can claim it by physically blocking him from entering the area with your body and stance. If all this is fear-based, try consulting with your Vet or pay for one session with a trainer. One session would not be all that expensive. (At least it wasn't for us).
Neutering is definitely in order.