You can't summon Paarthurnax, the summons is for Odahviing. You shout his name the same way you did when you called him to Dragonsreach to capture him.
The P-Dov has embraced the Way of the Voice and is, presumably, a pacifist. IIRC he helped you out with Alduin at the battle at the Time Wound but that's about the limit of his aggro.
Don't be disappointed, though, Odahviing rocks. He has not embraced the Way of the Voice, but sworn fealty to you as the Dovahkiin and slayer of his previous boss Alduin. Some of my best times late in the game have been provided by calling him for backup. It became my go-to method of dealing with large Forsworn encampments... just ride up, dismount, summon Odahviing, fire an arrow into the camp, find the best possible safe vantage point, and break out the popcorn.
It didn't hurt that I was riding Shadowmere at the time. Between her and the Big O, it was usually a hell of a show. Nothing like watching your horse chase your enemy down the side of a hill only to have him run right into the fiery maw of the dragon that just landed at the bottom of it.
