Do you think his work will be beginning on schedule? How willing is he to mitigate any risk he may expose you to? How realistic is it to quarantine yourself from somebody you write is there to help you (that is the impression I got). Do you trust him to limit outside contact? Just what will he be doing for you? You need to consider the risk factor involved in each of those activities and decide how to reduce that risk.
Depending on how high your risk is keeping safe may not just involve being isolated from him it could also involve disinfecting whatever goods he brings in with him or acquires during his stay. It may mean separating the dishes he uses. If you have a dishwasher his would be washed apart from your. Same goes for the laundry. That also means disinfecting the washing machine before your clothes go in. The bathroom would have to be sanitized after he uses it. Every. Time. If he is doing food prep he needs to wear food service gloves and yeah a mask.
I would also figure out distances and mark them. For example know where to place furniture so you are at least 6 ft apart whenever you might be in the same room. Is it feasible for one of you to be awake while the other sleeps? Sort of living in shifts. That will minimize contact.