Just see how it goes with the first one- how fast he digests & how he acts. There's no hard & fast answer- you're in the best position to judge by how he acts & all. Trust yourself- I do.
If you have reptile vitamins, you could add them to the rodent, but I wouldn't dust them on the outside, in case the different smell puts him off eating. I like to put them in the oral cavity (mouth).
I agree on the fecal test, once you're getting some stool from him. Fingers crossed- he's so lucky that he was found.