[targonrotts]

how old is your dog?
how many vaciines has he had?
did they run any bloodwork, such as a CBC?
do you have ticks or seen any ticks on him?
did he eat anything different-did they xray to rule out obstruction or only palpate?

those are some of the questions i would have to see what can it be.
tick diseases make them tired, slow, lethargic, can upset their stomach.
obstructions and parasites can make them the same too.
in other words, too many diseases start the same way.
if he is not better, i would ask for more precise bloodwork and possibly xrays.
as for parvo, if he is under 8 months and vaccines were not timely, i would consider the test.(bloodcount would usually be low).