Most, but not all, cases of scours are caused by bacteria in the calves digestive track. I would first take the calves temperature. If it is elevated, take him off milk (or milk replacer) and replace with a good electrolyte replacer, such as Bounce Back. Feed the electrolyte in the same way that you were feeding the milk. Start treating the bacteria immediately with a good antibiotic. There are several good over the counter choices or your vet can give you a prescription drug. Either way, treat the infection aggressively. These little babies can get real sick, real fast, and die with in a few days. Once the scours is under control, give his a squirt of Probios to get the "good" bacterial in his stomach going again and start back on the milk. They are usually better in two to three days. This would be a good time to get to know a good local large animal vet. Most of the ones I have known have been excellent teachers and were more than willing to give free advise in a situation like this. Good luck and keep us posted.
Throw a few sulfa bolus's down his throat right away- Most vet or good feed supply outfit will know what works best for your area. I usually give them a shot of LA-200 and vitamins while I'm at it too. Usually works if caught early.