You don't need to have him sign anything--just don't accept any rent (including indirectly--e.g. by having him pay utilities, insurance, etc.)--and he'll be a guest not a tenant; a guest may be made to leave at any time, without any prior notice. Note that if he does pay anything that could be construed as rent, he'll be a tenant, not guest.Alternately, let him be a tenant and pay some costs (e.g. the insurance; part of the mortgage or taxes; utilities; etc.) and give you a security depost against damage, but have him on a written month-to-month lease, so you could evict him on 30 days notice; this way, you can still get him out quickly if you need to, but could have at least have him cover some costs for you while there.