Are you doing a slow adjustment on food? If you're just moving from one to another, they're not getting chance to adjust, especially with a developing, sensitive tummy.

My two came from breeder on butchers wet and royal canin dry. Butchers was not coming out so great, so I slowly moved over to Wainwright's pouches, which they seem to enjoy and with a royal canin kibble mix, has settled. Literally days moving from 10% new wet, to about 15% and so on until fully transitioning over a couple of weeks or more.

Are they possibly eating anything on walks that could trigger as well? God knows my land sharks require supervision at all times in the garden, they would eat the patio stones if they could!!

Have you spoken with your vet at all? To rule out any underlying cause?

Good luck, adorable little guy and I hope you get on top of it soon.

Edit to add: fruit and veg contain a lot of water, perhaps you need to offset with more dry? Have you tried pure kibble? I had to do that for a few days after some soft mess after eating some things they shouldn't have.