What can I legally do if I am on the lease but my boyfriend is fighting me for the place?

The lease between you and your landlord is what governs your right to remain in the property.  Any standard form lease you signed will set out the contractual rights between you and the landlord;  only the named tenant (you alone) will have rights and duties under that agreement.   In other words:  You are the tenant on the lease who is obliged to pay rent, and in exchange you have the right to live in the apartment.  Your (soon-to-be-Ex) boyfriend, who is merely listed on the least as an added “roommate”, is not a co-tenant, has no direct legal relationship with the landlord, and likely does not have the legal right to take over the property at all.

In terms of what to do:  For now, the best answer could be “Nothing”.   Without more, the landlord is not entitled to break the lease and end your tenancy early, nor swap in your boyfriend in as tenant, merely due to the so-called “hassle”.  If your boyfriend decides to try to file a complaint under the Ontario Residential Tenancies Act, 2006, it is unlikely he will get far because he is only occupying the property at your whim; he has no rights as a tenant.