Oh my goodness....thank the universe this kiddo found you! It sounds like this little stray has found a great family
. Poor guy...this does sound like digestive pain, and that can be miserable. Marcel had this kind of pain as a pup, post-giardia.
I wonder if the vet would be interested in giving him something to soothe his GI, like sucralfate or...? Also, was an ulcer ruled out?
I agree w/ FuFu in terms of repopulating the GI tract bc it's likely very much out of balance. If you have a PetCo near you, they carry BeneBac paste - which is a high-potency probiotic gel - and this can really help at times, esp during stress/sickness. I always pick up the size that's in the small animal/ferret area of PetCo - as I like this size better for yorkies. You get 3-4 small tubes for about $4; I keep BeneBac on hand at all times, in my ER kit - it's nice to always have around.
I so hope he feels better soon!