Dear but what about a sublet?
You have a good question.
Many tenant attorneys would recommend that you not leave your ex in the apartment with a sublease requiring you to sublease for the time he needs to build a nest-egg to move. You might discover, at the end of the sublease, that he will not move. At that point, you could lose your apartment.
A landlord is not required to give consent to a sublease and may withhold consent as long as reasonable. Landlords have created a ritual to determine if they are obliged to consent to the sublease or may safely reject the request.
Most use a form of "questionnaire" supposedly to learn whether the proposed sublease is made in good faith, where the tenant will reside during the sublease and that the tenant intends to return when the sublease is over. Among the items sought in this inquiry period, are many that go to the ability of the proposed subtenant to pay the rent.
The process of inquiry may run for a while but the thirty day clock for the landlord to decide starts when the provides the full questionnaire to the landlord.
The great danger for you as the tenant, is that you need to locate your temporary residence, and go through that tenant's sublease approval process, in advance of first submitting your request to consent to your sublease to your ex.
You may be better off subsidizing his rent for a while in his new place rather than trying to give him time in your apartment and in your absence to get his finances together so he could move.
Whatever you choose to do, make sure that you have retained an attorney to get you through this process.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.
DO NOT leave your boyfriend in your rent-stabilized apt. If you have been there 15 years your LL would like nothing better than to file a non-primary residency suit. On the other hand your boyfriend depending on your past relationship can file for succession and claim that you abandoned it. Yes, you will jeopardize your rent stabilized lease. Even if you asked the LL for permission to sublet you may find yourself without the rent stabilized apt.