Can I ask what food he is on atm? I hope you dont mind me saying but 300g sounds a lot.
When Willow had colitis as a pup we, on the vets advise we put Willow on RC sensitivity control. This seemed to settle her tum really well. The vet advised if it suited Willow to keep her on it, however i am aware that most vets gets commission from RC and Hills. When I looked at the ingredients i wasnt impressed most certainly when considering the cost of the kibble. However it sorted her tum so that was the main thing. A good few months down the line I did a bit of research and looked at threads on here and decided to get a sample from barking heads, we gave her 'Fusspot' potato and salmon, good ingredients and a lot cheaper.

Weve never looked back since changing her to Barking heads in terms of her tummy.
Good luck, having a pup with a bad tummy is so worrying. Hope he is feeling better real soon.