Rape

A boy at the age of 13 did a heinous act.He penetrated penis into a girl's anus when she was 8 or 9..He told her that it was just a game...After that,he realized his mistake,told his mother and few of his friends in deep remorse and repent to Allah and ask forgiveness from that girl..Now he is 20 and recently knew about the punishment of rape and it’s compensation as a dowry..So,he is shcoked now as he didn’t knew that before..He is also confused that whether he reached puberty that time or not,whether it was a rape or not, whether he has to pay compensation or not and whether he repented from that sin properly or not so that he can move on and marry another woman...It will be so kind of you if you kindly clarify it in details..He is suffering from waswas for this.