Welcome! I am Dr. Altman, a licensed veterinarian and happy to answer your questions today.
First, congratulations on your new addition!
It is fairly normal to have a dog enter a new house with no clinical signs of stress until 2-5 days after adoption
Often they go home, act normal with normal stools/ appetite then within a few days have gi upset due to the stress of the new environment
It is nothing you have done but definitely would advise a veterinary visit within the next 24 hours with a fecal evaluation to ensure he has nothing that requires treatment. Most veterinary clinics offer new client/ adoption exams at no charge to assist in developing a rapport with you and your new addition
Tonight you can try cooking some lean meats and white rice to stimulate his appetite. If he goes 24-48 hours without food at his size he will be alright. You do not want to force the food/water because often they are refusing because they are nauseous and ingesting anything will cause vomiting
You can try pepcid 20 mg every 12 hours for a few days OR prilosec 20 mg #2 daily as an alternative along with the white rice and lean meat but if there is no change in the next 12-24 hours I would advise a veterinary exam before things progress to vomiting/ diarrhea
I will monitor for your entry to the chat to assist you both further, please let me know how else I can assist you today
Thank you for the advice! We will try cooking the lean meat and rice tonight and schedule a visit to the vet within 24 hours if nothing changes. I will be sure to let you know if I have any additional questions.