I would try just saying "howl" whilst he is howling...then treat. Maybe the 'good boy' bit is confusing him.
Also do you have the phone out? maybe he is using the mobile as the 'cue' to howl...try hiding it and then treating when he howls...
when you think he has the idea try giving the command 'howl' and then wait...if he doesn't howl within say 5-10 seconds use the phone and treat again, and repeat with the phone a few more times.
I'm not an expert - i tried to teach Oka to bark on command but she doesn't like too...she can but each bark gets successively quieter and then she looks at me like "really? y'know i'm not all that interested in this game" and then goes into her 'i'm going to' (sulk) position.
hope that helps