Have you considered a rear rack with a carrier attached? If your dog is the type to lunge at others, it might be wise to contain him inside a pet carrier strapped to the rear rack where it won't have as much of an effect on the handling of your bike. Otherwise, you might consider a trailer unless you think your dog will tear his way through the fabric during your commute.

My Havanese rides comfortably in a trailer, but he's also very quiet and calm during our rides.