I would get scour meds from you vet they give me a 3 day treatment 2 different pills and like 2 bucks oh and a pill pusher or be prepared to shove you hand way back in their throat the feed store has one of the pills I get from the vet kinda spendy though unless you will have lots of baby calfs you have to buy a big jug of them...I can't find the other in the feed stores so I just get from my vet.....also go to your local farm feed store and buy a gallon jug of keopectin....cost me 10 bucks a must to have on hand you can give it to cats, dogs, horses, cows, goats, and their babies and every time you see it poo runny or watery give it the required dose you gotta give it frequently....ask your vet what dose....the bottle may say the dose. Yellow poo is grain scours.....I give free choice sweet grain from 3 days old so mine are well use to it by 5 weeks old and the sooner and more hay they eat the fiber will firm up the poo too.
The meds I get from the vet are one was sulfa max and the other was albon 5mg I belive I have not had any babies for a while now....one of those pills works right away and one last for 3 days.....also jerseys are so weak and go very very fast. I would also get some electroylites in the little guy....I use corn syrup and water 2 qt bottle 3-4 times a day if he will drink it....they like sweet.....I also use gatorade and water 50/50 mix switch back and forth throughout the day. If not drinking you need to bag him or find someone who can a local farmer.....when their tummy is upset they don't want to drink.
If you have any queations feel free to ask.
Good luck let us know how things go!