Hello, welcome to the website. My name is Tony, I can assist you with this.
No, he cannot claim title to the land after grazing the sheep on it with your consent. The whole concept of adverse possession, which is where title is claimed, is that it requires the land to be used without the owner's concent. Hence, if you consent, then he can never use the period of possession/grazing as a means to claim adverse possession.
You should always try and get the person to sign something to confirm that he has your consent to graze his animals there too though. This should assist should he seek to claim your land later - it will demonstrate your consent, which means that his adverse possession claim must fail.