My 8 year old likes games and is interested in creating them, though he has something pretty specific in mind. I'm not sure what software (game kit, modding tool, etc.) could supply what he wants.

What he wants is to be able to create a world, and have the character go about it, find parts and build stuff from it, grow stuff and feed on it, discover creatures but not necessarily have to fight them (though they might decide to attack). He prefers it to be 3rd person.

This seems somewhat complex and specific to me, and probably best suited to an editor within a game that's already of this style, but I have no idea what that game might be, or what other options exist.

He did enjoy playing around with RPG Maker maps, and I installed GameGuru but he complained that it was geared towards shooters. I tried the Sword Coast Legends editor myself, but it's also not exactly what he'd want. I'm not sure if there's anything to do except try to create or buy suitable assets for Unity (for example), but that would be quite a project.

Advice would be appreciated.

His best chance is probably to mod Ark: Survival Evolved, although that game has been rated 16+ by PEGI so you may want to check it out yourself first.

Minecraft could perhaps work (never played it, not sure if it is appropriate). More serious programming could be Scratch, which is based on a "stage" where everything happens, with lots of graphics etc. Aimed at around his age, I think. Lego with some form of computers could do the job also if he's more into building than programming.