If the unit is not under rso, it sounds as if you will still have a challenge. I would suggest you give him as much warning in person/writing as possible. Someone who's been there for that amount of time will have a hard time leaving and finding a new place. I would also suggest you offer helping him find a new place and perhaps offering some relocation assistance even if it's not required. Sometimes being helpful is better and less expensive than the alternative which will be hiring an attorney and going through the eviction process..and if the unit has not be "livable" as you claim..you, not your tenant is responsible and he could be allowed to stay there and/or you would owe him money for overcharging rent...yes, it's possible.
You may want to consult with an attorney.
Be prepared but do it, it is necessary.
If you tell him in advance , he hopefully has more time to organize his move and empty out the place, which will be less work for you later. But he could also damage the place to take revenge.
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