I definitely wouldn't be worried about the growth rate, as they all grow differently even on the same schedule. If you're worried about his stools, you can always take a sample to the vet, most only charge about $30 for the sample to test for parasites, and usually they only need to see the animal if it's positive. If it's negative then you ruled out parasites...which would probably be my only concern. What do you mean lethargic though? Because I have some that are very excitable and curious and others that could care less, but lethargic definitely isn't the word I would use...