Obviously, getting assistance from the vet is the best approach. However, you seem to be experiencing the same thing that Alice and I run into time and time again as vets in our area do not get too interested in bottle calves and we are most often left to fend for ourselves...
Based soley on the information provided here, I would suggest you give the calf antibiotics on a scheduled basis (depending on your choice of med) however, do not stop the antibiotic at first signs of improvement. You want to ensure you have completely knocked out the illness. I personally like banamine as it keeps fever down, makes them feel better, which helps with their eating. If they will keep eating, you are way ahead of the game! Give a big shot vitamins A/D/E and B complex...will help as well. Get you a tube of probiotics, as suggested, and give it to him at each bottle feeding.
You indicated the calf was in a pen alone. Make sure the pen is clean and dry. We use calf hutches with a panel in front, so the total area is small, which requires continual movement to keep the area clean and dry. Ensure the calf has clean, fresh water. AND get him as far away from the others as possible. As previously said, this one will possibly infect the others.
If you are having any sunshine during the day, ensure the calf is out in the sun and not laying inside. We will close off access to the hutch if weather is good and force them out in the sun.
I am not saying this is the gospel...just what we would be doing were we in your situation. As the old saying goes...been there, done that.
Good luck and keep us posted.
Van
