Dog sick after switching food?

Hi there, hoping that someone can help me out. I just recently got a puppy (he is about 7 months old). About five days ago we started transitioning him to a new type of dog food. Almost right away we noticed that his stool was getting a little bit looser and eventually it started coming out as a more pudding texture. Two days ago now we ran out of old food and could no longer slowly transition the dog. This is when I started to notice that his stool was getting to be more watery. He had trouble sleeping last night because his stomach was so upset. I did go and buy a bag of his old food yesterday hoping that that would help, I fed him the old food for all meals. He woke me up about three or four times during the night because he was whining and one time when I got up with him after going to the bathroom he vomited. I have been feeding him pure pumpkin as well as making sure that no one feeds him scrap food. I started to notice the loose stool about 4 days ago now. I am wondering if I should be taking my pup to the vet or if I should follow other advice that I have read and withhold his food for 24hrs before feeding him bland food like rice. Any helpful advice is very much appreciated.