First of, make sure you have permission off the farmer, to walk your pup threw sheep, as going to a field with no permission from the farmer, could have your dog shot!!
Then, walk him threw sheep on the lead and every time he looks at the sheep, have a rolled up news paper with you and give him tap around the nose, a shock with a sharp NO!, keep walking him threw sheep regular, he will soon get bored of them, after a while you can let him off the lead and see how he goes.
Also, regular refresher courses walking the pup/dog threw sheep, is a must, as they can forget after a while and in the