Which Oscar's type is it? Oscars pet food - Review from Pet Food Choice
Some of them here... to be honest all of them actually, don't look too great.
You do know that "maize" means corn, right? Many people believe corn is not the best thing to be feeding dogs. I don't think it is terrible, but I would not want it to be the major ingredient in my dog's food. Same thing with wheat.
I don't know what the rules are in the UK for dog food or labeling, but these food seem to lack vitamins. The foods in the USA tell you the vitamins in them.
Honestly, they look pretty low quality. Their "working dog maintenance" ration only has 16% protein, which the same as chicken feed I give my hens. (A chicken's job is to turn low protein food into high protein eggs and meat... a dog is more of a carnivore/omnivore than a chicken.)
All that said, I'm not sure these foods would upset his tummy, as often that is caused by too much protein and fat, but it is possible the ingredients here just don't agree with him. Perhaps your pup does not do well on all that corn or wheat?
In any case, I cannot imagine raising a growing pup on such a low nutrition food, at least not without supplementing it with some canned meat and some vitamins.