I would have him in a stable and have a rug on him with both a back strap and leg straps and let him stew. By the end of a few panic sessions, he should be use to it.
I have had a couple of horses like this and both had 'back problems. Could be something to do with that.
I would also put a crupper on him and hang all sorts from the strap so it is all behind his girth area. Basically sacking him out with what he is afraid of.
I have sacked him out so much on the floor that I'm fairly sure a bomb going off behind him would only warrent an ear flick, certainly a bird scarer gooing off 10ft from him no longer warrents even a lifting of his head from the grass.
I'm asking for ideas on how to do the same with someone in the saddle without putting the rider at risk