I was lucky, my dcs were up for every residential trip but then the school didn't take them on overnight aways until Year 6 and by then, they were already used to Cub Camps and the like.
In your case, OP, I really can't see any great benefit in making your ds go on this trip. If he is unable to use the loo on his own in his own house then he's certainly not going to cope with being away from home and the likelihood is that you'd just end up going to collect him anyway.
I had two friends with equally reluctant dcs and both attempted the "suck it up" approach despite their dcs determinedly expressed reluctance. In one case, the child so detested the residential trip that he refused, point blank to go on another one. Even in secondary school and even if it was then part of his GCSE course.
In the other case, my friend ended up driving hundreds of miles to collect her dc. Who had been so homesick that he'd needed a member of staff to look after him pretty much 100% constantly. Which didn't make for a great trip for everyone else either. Both now wish they'd not pushed their dcs but been lead by their very real wish not to go on this trips.
What I would do, is attempt to build up your dcs confidence a bit though. But gently and without the need to be away from home just yet.