wait the 7 days and transfer it back. he cant use his id and you can use yours until you transfer back. assuming you didnt transfer to a new 3ds

 

He currently has his ID on both systems so he should be fine once I remove it from my system no? We can't really wait a week as he's going away - surely there's an unofficial way around it?

 

The unlinking needs to happen on the server side, you can try contacting Nintendo and getting them to transfer the NNID over for you (come up with a lie like for example you lost or broke the 3DS)
If you system transfer back, your NNID will be transferred along with his, so the only option if you both want to keep your current NNIDs is to contact Nintendo. Though they might transfer both NNIDs.