Hmmm, how old is he? So, his poop is totally normal? But, he's not really hungry? Is his tummy distended/swollen?
Check his gums - are they nice and pink? He is drinking water? Energy level/alertness okay otherwise?
Was there a storm or anything the night he was alone?
If it was his anal glands, he'd likely be rubbing his bottom on the floor - and his poos would probably be funny.
I guess at this point, I'd try stimulating his appetite with something he LOVES like peanut butter, bits of cheese, cereal, bread, crackers, yogurt, cottage cheese - anything that he likes. Hold him, comfort him while he's getting these treats.
Palpate him ALL over his body to check for sore spots/tenderness. He may have been just scared by something - but just triple check that there is not injury.
If you can't stimulate his appetite though, I'd probably take him in within a day or so (unless, of course, you suspect injury).
Welcome to YT, btw - sorry it is under these circumstances, but glad you found us.
