While he is proud of his son, who has demonstrated himself to be a perfect heir, Risei fails to understand Kirei in any sense. He believes that his son, who has recently lost his wife of two years, may be able to find some purpose in life again by aiding Tokiomi during the Holy Grail War.
Both Risei and Tokiomi, upon discovering that Kirei's command mantra had appeared three years before the start of the war, both came to believe that Kirei was destined to provide Tokiomi with an additional Servant and associated command spells for the upcoming war. Without informing Kirei as to the true nature of the mark that appeared, they transfer Kirei from the Church to the Mages Association as Tokiomi's pupil. Neither man reveals to Kirei that he can also obtain the Holy Grail, instead telling the young man that he is performing a mission for the Church.
SpoilerAccording to Kirei Kotomine in Fate, his father died trying to protect him. As revealed in Fate/Zero, after Caster's defeat, Kayneth Archibald El-Melloi approached Risei to obtain additional command spells. After Risei reluctantly gives what Lord El-Melloi has come for, the Master shoots Risei and leaves him to die. As he dies, Risei leaves behind a message containing a secret password that will provide his son with all the remaining command spells Risei could give out. Kirei arrives to find his father dead and his father's final parting gift, and comes to the conclusion that Risei died trying to protect him. Though Kirei sheds tears realizing his father cared about him till the end, he finds himself feeling the same feelings he felt when his wife died - the desire to have seen them die.