Stuck in a bootloop with no way out.

Hi, this is my first time asking a question here so I'll try to be as detailed as I can and before I ask I have to mention I've done hundreds of successful repairs on various iDevices so I know the general basics e.t.c.

Basically my Nephews 5S wouldn't boot and would just loop constantly, I suspected it was a failing battery so went ahead and replaced it, but it's doing the exact same thing, bootlooping with a red screen every 4th-5th loop.

Now, I'll get the possibilities out of the way of all the things I've tried;

- I've re-seated the LCD assembly, no difference.

- I've removed the Logic board, double-checked all connections, no difference.

- I've used the battery from my own 5S (genuine battery), no difference.

- I've tried swapping the LCD from my own 5S (genuine LCD), no difference.

- I've tried restoring via iTunes, it errors when it attempts to start the restore process.

- I've tried restoring via iTunes when there is literally just the Logic board, battery and power ribbons attached, same error message as before.

- I've tried restoring via DFU, same error message.

- I've looked carefully under a magnifier and I haven't damaged any capacitors around the battery connector (I always take care removing the battery because of this fact).

Basically I am absolutely STUMPED, I've tried almost every variable there is when it comes to troubleshooting this thing and for the first time in a long time I am completely out of ideas and I hate being defeated, is there anyone who can offer any kind of input or their own experience at how they managed to overcome this?