not look at him, and he does not have your attention if he cannot see your face.
give your dog a treat as soon as he quits howling and praise him for being quiet. rewards for quiet behavior breaks the attention howling cycle. wait for a few seconds longer of quietness before giving him treats each time he stops howling and eventually do not reward him with treats but petting and praise for silence.
throw a toy for your dog to play fetch with if he is howling in response to sounds such as a siren or other dogs in the neighborhood that are howling. try to get him to focus his 