My puppy's poop is firm, but soft at the end. What should I do?

That depends on how soft it is and how frequently that is happening. For some dogs, their ‘normal’ is firm in the beginning of the stool and then softer (but still formed) at the end. If every thing else seems to be normal and you don’t see any problems with the whole stool being soft/loose and no liquid/unformed stool at the end, you may do nothing and just monitor for now. It may also be beneficial to take a photo of the questionable stool and post again so that we can see what you are seeing.
I assume you have done all the parasite testing (including giardia) and routine deworming to rule out parasites? If I have a patient who is continuing to have a softer stool, I will often treat them with a 5 day course of Panacur just to be certain no parasites remain.
I agree that if the metronidazole does not seem to be making a huge difference, I would probably shy away from that. If I suspect a bacteria cause to continued soft stool, I will often treat for 6 weeks with an antibiotic called Tylosin or I will treat for 10 days with a sulfa-type antibiotic.
Lastly, a continued soft stool could also be related to food allergy. The diet you are feeding is excellent. It’s meant to break down easily and decrease the work load on the cells. This diet is very good for dogs who can’t handle the richness of most dog foods. It is not however formulated for dogs who have food allergies. That is a different immune process. Don’t be tempted to put him on a grain free diet. Most dogs who do have legitimate food allergies are not allergic to grains. Instead know for sure if it is a food allergy. The diet I recommend is made by Royal Canin and called hydrolyzed protein adult HP. Since Bentley is a boxer puppy, he is still growing. This diet is formulated to meet the requirements of growing puppies so it will both help with food allergy as well as support normal growth of his bones and soft tissue. You would conduct a food trial on him which will last for 12 weeks. The stool is normal at the end of 12 weeks, then you know it’s an allergy.
I recommend to discuss any of these options with your veterinarian if the stool is still abnormal or if Bentley continues to get problems with a loose stool in general. I hope this is helpful and please feel free to post any additional questions!