A small rent or tear in the anus could allow a bit of bleeding but blood produced that way would be obviously blood, bright red. And it would be only a drop or two. Uncomfortable for kitten but not an emergency.
Be sure to feed only small bits of food at a time so as not to overwhelm his little tummy with more at once than he is used to and because it is a new food. No milk, in case you didn't know that.
Internal bleeding would produce dark looking stool.
If all else checks out OK, pink gums, hydrated enough, I would feel comfortable leaving the VEt visit a day as you have no choice but to do. Kitten probably has worms and fleas. I'd take that bit of stuff that concerns you along with a fecal sample to the Vet with me.
I am not a Vet so of course if you are in doubt or worried VEt immediately if you can.
Poor wee thing. Do you think there were others in the box too? And maybe they skittered away and hid?