I think best would be to board horse in boarding facility and lease him from there. This way you can also get input and follow-ups from BO. Plus the feed won't be changed for sure. And I agree: get contract with everything you can think of written down such as expenses (deworming, farrier, vet bills, emergency calls, etc.), WHO can ride a horse (only the leasing person, that person and his/her kids, anyone they want...), can be horse taking on trailer ride from barn or not and if so on what conditions, etc.