Are you feeding Fishmonger's Finest Adult? That review is here.
It has 32% potato plus beet pulp so maybe these are contributing to the bulk and softness of the poo. However, your dog has had multiple pathogens and is quite young so it might take a little while longer for the digestive system to settle down. My friend's dog has recently had campylobacter and the poo is still soft at the moment, even though treatment has finished.
Looking at the review, there are some advantages to leaving things as they are:
* The recipe for this food is quite simple and fish is a good, digestible source of protein.
* You mentioned joint health and this food contains glucosamine and chondroitin which is added specifically for this.
* Changing food when a dog is, or has been ill is not a good time. It could make things worse, at least in the short term.
* Overfeeding - the symptoms you describe are typical of overfeeding so make sure you weigh the food accurately and be aware that the recommended daily allowance figures are often too high for some dogs.
* Pre/probiotic - start him on a good pre/probiotic. Perhaps something like Lintbells YuDigest might help.
Regarding the protein, it sounds right for the age of your dog. There is a thread here
about feeding large breed puppies. There are other ones too and these can be found by looking at the bottom of that page (similar threads) or by using the search box at the top of this page. Your pup still has some growing to do and needs good quality protein in the right amount. This is particularly important in view of the fact that he has been so poorly.