Oh NO! poor reuben - sorry I haven't seen this sooner - have been away so just catching up on everything. I remember the explosive blood filled diawotsit when Jarv had HGE (he didn't have a blockage, just HGE), so I fully sympathise with you and Reuben. It's awful to see them like that and in so much pain.
If it's any consolation, if it IS only Haemorrhagic gastro enteritis than once he's re-hydrated and the anti inflam's for his tum (and the anti-b's) kick in I'm sure he will recover very quickly
Fingers crossed he's on the road to recovery today