Right to Rent - length of tenancy

Tenancy Types and Management

I have a potential tenant who is American but has a St Kitts passport for tax reasons. He currently owns a property in the UK which he is selling but wants to keep a base here for when he visits. He wants to rent my property for two years (paid in advance) with a view to renewing annually thereafter for five years. However, his immigration status means he is restricted to how much time he can spend in the UK (6 months). I'm getting confused by the right to rent restrictions in terms of the length of the Tenancy Agreement. Is it unlawful to enter a two year contract with him given he doesn't have the right to remain in the UK for that length of time? Even though he would abide by his visa restrictions and leave the property vacant for the rest of time.