When you first go to the settlement of Finch Farm, you're asked to find one of the Finch boys in a factory just north. An optional part of that quest is to actually rescue him while defeating the forged. So I rescued him and returned to the farm, and Mr & Mrs Finch thank me and I can use Finch Farm as a settlement. I know one of the brother's names is Daniel, but I can't remember if it's the one I saved, or the one who remained.

Anyway, the brother that I saved just stands there, won't do anything. I can talk to him, but he just thanks me for saving him. I can't equip him with weapons/armor, nor can I assign him to a task. Mr & Mrs Finch and the other brother are fine, they can be equipped and assigned. It's annoying because I know he's going to get his ass killed in an attack which is sure to occur in that area. Has anyone else experienced this, or know a way to correct it? Leaving and returning doesn't help.

Must be a bug/glitch. I have him running my mortar cannon.

Only one Finch brother is in Sangus.
That other is simply named "Settler" and not one of the Finch family.

Giving this settler gear, ammo, and a weapon, usually off the dead raiders for me, seems to get his courage up enough to leave the Forge area.

Jake (the son you rescue) did that for me, too. He just kept repeating the same thing over and over. I just went and did some other quests and kind of forgot about him. When I came back, I was able to interact with him and assigned him to guard duty. It took a few in-game days I think.

The family that lives there are Abraham, Abigail, Daniel and Jake (if you rescued him) and they say some pretty funny things, if you happen to listen to their conversations.

Yeah, just give it time and he'll go back to his normal settler state.