Purchasing building, long-term tenant has no lease

[Forest K.]

I'm about to take ownership of a four-unit property in Massachusetts with my partner. Neither the owner nor tenant have a copy of one tenant's lease. They do have signed confirmation of the tenant's security deposit amount. The tenant is now month-to-month and we're not really concerned about him trying to do anything inappropriate as his track record is OK (besides needing to remove some of his stuff, including an unregistered car, from the yard). I'd also prefer to keep the tenant month-to-month for rent-raising-flexibility because he's paying well under market rate. But my partner and I need some time to pull together financing for renovations and we don't want to risk having him leave before we secure it (which may be up to a year). 

Do you have any advice on how to proceed? Is it legal or advisable to have a tenant with no lease? If so, are we then required to put him under some contract? The other tenants' leases are identical across a number of years, so it is assumed that his lease is the same. Do we recreate the old lease based on the others and ask him to sign it as a replacement?   

Thanks!