Poor Bailey - take a look through my older posts for loads of advice from others on poorly tummies! Rosie ate grass quite a lot while she was poorly. I still don't actually know if it was a cause of the illness, or a response to it, but personally I think it was some sort of response. It never made her sick, but maybe it did something to make her tummy feel better.
It's a bit horrid, but check his poo for blood and mucus (just looks like slime) - this is colitis, which is what Rosie had. You have to be careful, because their intestines are so tiny, the vet told me it can actually get wrapped around itself and this can be a big problem. Rosie was always bright and lively so the vets were fairly relaxed about it but told me to rush her in if that ever changed.
The things that worked well for Rosie were Activia yoghurt (a spoonful in the evening) and finding a food she was okay with and sticking to it (using it for treats too - not giving her ANYTHING but the food and the yoghurt - and water of course!). The food that worked for Rosie was James Wellbeloved Ocean Fish and Rice.
Personally, I think the antibiotics she was given in her first week (for kennel cough and colitis) didn't help much at all and may have made things worse, and I wish I had questioned the vet more. Giving her the yoghurt marked a big turning point for us and I still buy some regularly to keep in my fridge just in case!
I hope you don't need any of that advice though - lots of puppies just get a bit of runny poo from the stress of moving!