How old is the puppy? What food did they send with him?
Your pup has been through A LOT. If he is not eating, that will definitely affect his energy level.
Where and how are you feeding? There might be a lot of stres around feeding now, making it hard for him to eat. Then the tummy gets upset, and it's a vicious cycle.
I had a similar problem when I adopted my girl. I just left food out for her, which she eventually ate on her terms. I found that when her tummy was upset, some chicken soup usually worked, so I always had the organic stuff on hand for or, or made some from scratch and froze some, so I always had some for her. For the diarrhea, I really like probiotics.
It can take a good couple of weeks for them to settle in, and it's hard to tell exactly what is wrong here, if it's nerves or fear or something else.