I'm very glad you have a vet appointment set up for him already. The vet will be able to examine and determine if this is just an adjustment period or if there is a medical problem. Two things that came to my mind were Parvo and Twisted Gut. I lost a puppy to twisted gut before and the symptoms were similar. I don't say that to scare you, please don't worry until you see the vet.
We recently got a new pup also and the first, oh, maybe four or five days were kind of like that but she is fine now and we chalked it up to being seperated from her mom and from her siblings and in a new environment adjusting to new food and surroundings as well.
Your new baby is very lucky to have such a concerned owner and I do hope it all turns out for the best.
Please update after the appointment tomorrow! I will be praying for good news.
