was the calf 'off' the milk for any amount of time, say 12 hours? Then did he glutten out? could be the start of Crypto...I say could be...
could be the start of scours, just not seeing the signs yet.
sulfa drug works well
Is he coughing when drinking the milk? Sign of possible pnemonia
any number of drugs work well
did he get chilled?
might be a good idea to check his navel. Is it hot, is there swelling in the joints of the knees? Is there swelling in the navel area? can you still feel the inner cord...thick like...start of navel ill
might be good idea to give some electrolytes just in case and to rebalance his system if it is out of whack a bit
edit...have you seen it poo