Haven't had this problem on all my other saves...
Yes i have reloaded a save and tried again, but he is still over-encumbered.

Anyone know a console code on the PC that can fix this? Or any other methods?

Have you opened his inventory and taken what he has? Even if it only shows one, it might be glitched and have multiple occurrences in the inventory weight.

To that end, try giving him one thing then taking it back if he has nothing in his inventory.

Now, I have not adopted a kid yet, so I don't even know if they have an inventory. If not, then ignore my post.