I’m no expert – but I Am going through stomach issues with my guy too.other than seeing vet first ( as you have) Cutting out the veges is def the way to start , then I also cut back the pellets ( if it is a sensitivity related thing then cutting back in the rich fibre in pellets can help ) making sure he has plenty of hay and water. With my guy these things settle him very quickly ( we can’t do green said at all for him )
Any reductions in his water / hay intake , any larthargic signs at all , any increase in the diarreah ( this is presuming he is only doing runny cecotropes and is still producing normal round poos too?? If not I would refuse to leave the vet )
Then we would need seen immediately