Roblox is your best bet! Play to your son's interests rather than starting from scratch. They have a scripting language which implements in Lua code into the games. He can start out by making his own items / weapons, and then move on to constructing a new game for Roblox. Get yourself an account and try it out! It's juvenile, but a little bit of fun at the same time.

Minecraft mods are a little more complicated, and do not offer the same level of ability to start small, and gradually build skills.

Show him your lack of Lua knowledge, and show him how you learn - he won't be very interested at 7, but seeing that you don't know is an important realization that will likely help him become a self-reliant tinkerer within a few years. If his interest is there, he will try to do things on his own eventually, but at first, copying what you help him with will be the majority of what he will do.