I have had many Americans hunting with me, and apply for their permits to bring weapons into the UK.
However in answer to your question. As long as you are the registered owner of the rifle and have permission on the land, and you are with him at all times, he can use your rifle without a visitors firearms permit. However he cannot purchase ammo, or be with the rifle without you actually being with him.
If he is not with you and he uses your rifle, you will need to sponsor him for a Vistors Firearms Permit, and also if he intends on bringing his own rifle over.
However from an insurance point of view if something, god forbid goes wrong, you might have a problem. I have always applied for a permit for all my clients whether they use my rifle and I am with them or not.
ATB
Sikamalc