Firstly, he is your cousin, not your brother in law.
Secondly, you and he were both kids, so there is nothing that you can do because it is not like he was an adult, neither were you, you both were kids. At best you can confront him and ask him to apologize or you can just move on from this and try to forget him by trying to avoid him or both of you don’t cross each others’ paths. I don’t think anything else realistically can be done.