bland diet? I would start there - boiled chicken or hamburger & rice. I would also add water to the food so that he stays hydrated, that is very important. The other thing I would reccomend is a probiotic and ASAP. Do you know if she is currently giving him any sort of probiotic? I'm not sure what you have access to in the UK - but I started Tyson on this: which I picked up at our local natural pet store, and I saw improvement in almost a day or two.
This won't hurt him at all, it goes in and re-establishes the good bacteria his body needs to help fight off the bad. This will also help his body absorb the protein from the food. And it should help with his diarrhea. I would try to get something like this ASAP if he' isn't on it now, and start it immediately. I hope & pray that all goes well! We'd love for you to keep us updated on his progress.