Oh poor baby, yes, he could have wet tail, but I did some research before answering and I seem to be finding yes and no answers about whether mice can have wet tail or not. The consensus seems to be that at the very least, the diarrhea, which could only LOOK like wet tail, is still a bad thing. I am not as familiar with mouse medical issues as I am with gerbils and rats but wet tail could be possible. The diarrhea and bloating are probably making your mouse a pretty miserable fellow and he needs some help fast.
He is not eating his toilet paper roll though...mice and gerbils and other small animals like to chew on carboard and other materials but it is not something they actually ingest. They are just using their gnawing instincts to keep their incisors whittled down.
If a small animal has diarrhea it can also be caused by eating too many veggies or fruits, which have a high water content. Some fresh fruits and veggies are a great supplement to a small animal's basic diet of lab blocks but too much can cause diarrhea. Wet tail is of course a totally different thing but can have the same symptoms. Stress can also cause diarrhea. Does your mouse have plenty of hiding places/boxes and places to curl up in and do his sleeping and nesting so he feels safe? Does he have plenty of toys, shredding material for his nest (like paper towels, etc) and not a high noise atmosphere that might stress him out? A clean cage, clean bedding and fresh drinking water in his sipper tube are all essential as well to his well being and comfort.
Did the pet store offer to examine your mouse for wet tail? Depending on the store and how long you have had him from that store, some stores have it written into their guarantee that they will have their vet look at your animal (or better yet, do you have a local small animal veterinarian who can examine him?) I have actually never experienced the process of having a pet store resident vet examine one of my pets as I got my rats from a breeder (my first two boy rats) and a local rat rescue shelter for my current two boy rats, so I am not sure what kind of policy PetSmart has about guaranteeing the health of your mouse. What did they tell you on the phone when they said your mouse might have wet tail? Did they offer to look at him or offer you any advice at all as to what to do for him?
One more thing...since I am not as familiar with mouse maladies, there might be some other people on the forum who could see my post and say, "Vlad, you are jumping the gun about a vet...he could just have some diarrhea from eating too many fruits/veggies and if so, stop the veggies for a few days, make sure he has plenty of water and maybe give him some bland type foods such as cooked brown rice/bread and his diarrhea and bloating should clear up just fine." That is possible, depending on what he has been eating. Hopefully someone who is more knowledgeable about these things will see my post and clarify a few things. I guess I just hear the term "wet tail" and it scares me because that used to be a dreaded word when it came to my gerbils and I was always thankful they never got that! I just want your little mouse to be okay and not go downhill.