Since the other commenters have mentioned a ton of great tools, programming languages, SDKs, and the like, I'll make some suggestions of a different type. You

Since the other commenters have mentioned a ton of great tools, programming languages, SDKs, and the like, I'll make some suggestions of a different type. You said you'll be helping your nephew, but consider this as well: find a local group geared towards helping kids learn to program. For example, Coder Dojo(https://coderdojo.com/ https://coderdojo.com/) has hundreds of locations all over the world. Your nephew doesn't need to be accompanied by a tech-savvy guardian or even bring a computer, usually such groups supply coaches and computers. They are a ton of fun for all involved, and it's much, MUCH easier to learn something challenging like programming with a coach or even other students.