If you need to make a sheep costume start by getting a lot of cotton. You will need to take clothes that are his size and than create the sheep look by sewing the cotton onto the clothes. Once done add a cotton tail and get him some white shoes and you are done.
I would buy a fluffy, soft white fabric and creating a pattern for a jumpsuit. I would sew a puffy tail on the butt and give him a headband with ears. Cotton balls on his shoes and maybe cut a mini fence out of cardboard for him to jump over.