Its hard to say if its normal, most days my pup very energetic and other days likes to sleep a lot, depends on her mood and what we were doing that day. Some things to check for as well are is he eating as before, nose cool and wet, poops are normal and so on.
The worst case scenario I had with loss of energy was when my pup had parasite infection, but she had watery diarrhea all the time and I guess parasite were taking away all the nutrients from her, so she had decreased energy but after treatment went back to normal.
But also remember they are not robots, so there is always chance of mood and energy fluctuation. But of course if it goes on for a while and you feel concerned, better to see your vet.