If you feed him element, he goes beast mode, and is rideable. But be careful. If you feed him element too many times, he'll be beast mode and agro on everyone, including you and tribe members, as well as on enemies. Once done throwing a tantrum, he will reset his relationship with element, and the process will start all over again. I would advise bringing freshly tamed ones to raids, however, the closer he is to his tantrum, the stronger he is in beast mode. It's a fine balance, so be wary if you do feed him element.