Okay, basically yes, that can happen, Porn is like a drug, once you start you really can't stop.
Sometimes when men do watch porn they don't get hard, but for us its like watching a normal 'youtube' video.
Its a bad habit and its damn hard to stop, so if your husband is trying to stop, the best thing to do for you and him is to, go into your internets parental settings and change it to block all pornographic websites.
This is just to help you if you want him to stop, and porn pretty much doesn't mean anything to us...and most of the time we do feel bad afterwards...its also a way for men to release tension without bothering anyone, or lashing out, kinda
also maybe he doesn't want to sound 'needy' towards you...