I'd start by finding an open source project and contributing to it or find a local developer community to help with mentoring. The language is almost irrelevant, as long as it isn't C++. The most important thing is a project he cares about and the mentorship of other developers to teach good habits.

Getting involved in Cloud Native Computing Foundation, https://www.cncf.io/, is a great place to start for both beginners and experienced pros. They have many ongoing projects. They have an annual conference called Kubecon that is also sponsored by the Linux Foundation. Containers are the platform to be in now, and many companies moving in that direction. Java still seems to have a following, however, I have seen many moving to angler and python for infrastructure-related coding. Honestly, the best thing is to try a few different languages and stick with the one you like. For hands-on, I think CNCF is the way to go and get involved in projects. KubeCon 2022 is 10/24 - 10/28 in Detroit, still time to register.

@steve773 Sorry late to the party, it depends on what he is interested in. I've mentored a bunch of young kids from 12 up and would be happy to talk to your nephew. If he is interested in AI/ML python would be a good language to start with. But again first find their interest, my daughter was interested in AI/ML (because she thought she would make a lot of money) but she was also concerned about a cousin who is deaf mute that was living with us. She really wanted to help out and she came up with the idea of an app to translate sign language.

Another thought is to have him look at https://summerofcode.withgoogle.com/ I was a mentor for an open source project many years ago. Just have him find something that interests him.