Can you use a bath or trough to wash him rather than a hose? I usually try to snip or wipe as much as poss if my dogs have diarrhea and then bathe only if it is really bad (or I cant stand the smell). Then I carry them in to the bathroom and they have their own non-slip mat for the bathtub. My dogs are big on eating wildlife poo so my male in particular often has accidents from upset belly.
With the fear induced upset tummy - I had a similar problem at easter while at my parents. Last time the dogs stayed there I went away for 2 weeks and my male was devastated. This visit he kept panicking I would leave him and if I went out even for a short while he got diarrhea or ate grass and vomited. By the end of the week he had settled down so I guess they eventually get over it.
To get my dogs used to the bath I smeared peanut butter all round the rim. I also use a shower hose attachment on the tap rather than filling the bath with water, then I can wave it around their bottoms.
I have found that if the dogs can stand on your body while you shampoo them, they are less fearful, because they aren't so deep in the water the whole time.