I think this grade A AHole is now starting to try and position things in his favour and you need to shut him down quick smart. I suspect his defence is going to be something along the lines of "your ECU being already defective or in state you didn't declare (bluetooth connected) despite being warned by him to do so". Remember small claims courts are not car experts so may fall for a pack of lies easily.
I think as a first step you should write to him telling him you hold him solely and completely responsible for what has happened. A good strong paper trail is what you need now for the small claims court etc.
I would start by writing a mini war and peace outlining all your dealing with him, phones calls, emails, face to face time and dates as well as content. Make sure you get in to it that you were never warned this could happen, let alone that you need to take any precautions and he should have made sure everything was good to go.
Once you have a detailed chronology send it to him and just make sure anything from now on is email etc so there is a written record.
This guy sounds very fly by night and may not actually have liability insurance in place. Is he a one man band?
His attempts to deny accountability are just plain ridiculous. Can you imagine a garage who you have taken your car to do new front brakes then trying to say "its nothing to do with us!" if a front wheel fell off on the way home