Sounds like you need to keep him warm inside your shirt next to your skin until you get to the vet. Maybe he's fading a bit, so might need some supportive care and antibiotic. Perhaps tube feeding. Did he get any colostrum? Does he sound raspy, or gurgly?

I had a one day old pup with diarrhea. The puppy was 2nd born in a large litter and ate too much. Her stomach was huge and the diarrhea resolved in a day. This might not be what's happening with your pup. Hope everything is ok. Good luck.

Not 1 day but closer to 6 days -

If it continues:

I gave Bene-bac for 5 days and it cleared up. It is safe for new borns. Talk to your vet about dosage.

1 day diarrhea is usually due to too much fat in the mother's milk. Not a bad thing. it'll go away as the puppy's GI track gets used to it. make sure he stays warm. agitated but not hungry sounds like a healthy puppy that had enough to eat.
oh and your vet won't know that unless you got a repro or breeder vet.

I am sorry to have to tell you that the puppy had an intussusception. It is a very difficult surgery on an adult, and really not possible on a day old puppy. Now I will try to keep Abby happy and healthy after losing her singleton.

Joan:

I'm so sorry for your loss.

Linda @ Staghorn

Oh how sad. So sorry for your loss. Prayers your girl gets thru this soon.

Very very sorry on this outcome. I was away for a couple of days and just saw this.

Thanks to everyone who wrote here and privately. Losing a beautiful little puppy is very sad and I appreciate all your support.