Congrats on your new family member, and welcome to YT!!
I agree that he's probably quite stressed with all he's been through in the past little while, especially being so young. In my experience, when they are stressed, it does have quite an effect on their bowels...usually diarreah, but sometimes the opposite.
Have you had him in to the vet yet? It's always a good idea to bring your new pup in to have him checked over just to make sure that everything's okay. It's a good opportunity for the vet to familiarize himself/herself with the new little one.
Do you have any pictures yet? We'd love to see your little guy.
Best wishes!