Now, let me give you a timeline of the different characters and let’s see if Kashin Koji fits into it or not. Assuming that he his Jiraiya’s son. In my opinion and judging from the looks of Kashin Koji, he looks younger than Kakashi Hatake and older than Naruto Uzumaki.
Now, I think that he is at least 8-10 years younger than Kakashi and 8-10 years older than Naruto. So, when Minato Namikaze was killed at that time he would be around 8 years old. Now, how did he learn Rasengan and other various techniques? We know that Jiraiya was gone from Konohagakure for a while and I do not think that he was alone, it is in this period that Kashin Koji was trained by the legendary Sannin and his father and thus right now is easily one of the strongest characters in NarutoVerse.