I've owned many kittens, so maybe I'll be able to help you. 
First of all, if it's still a kitten, it may not be ready just yet to move on to milk. But if it drank milk already for it's real owner, then maybe you should try what ColDeST wiNTer suggested by gradually adding water to the milk. Before you make any decisions about what to do though, it may be a good idea to talk to his owner and see what they say because they may be able to help you. I hope this helps! And good luck with the kitten!
