I don't fault him for not wanting to go anywhere so this needs fixed.
First things first. I highly reccomend you do what is told to you by the good folks here or have your Father in Law or whom ever is working on it come here and read everthing first hand. These things are tough enough without loseing anything in translation.
Pulling the break a way cable sounds like a good way to test from junction box to brakes.
If it passes that test you need to check from junction box to controller. I would start by diassembling both sides of the plug for inspection,clean and tighten wire clamping screws.
Chances are if there is a loose conection,it will be discolored due to high amp draw of brakes. Next to junction box where plug pigtail mounts on trailer.
It's possiable but itermitint controller failure is very unlikly.
Pulling a friend's trailer or loaning his trailer(with a caution of course)could bring it down to almost certainly controller with possiability of truck wireing. Controllers are almost plug and play so borrowing before buying might be an option.