To play devil's advocate: The landlord living in the house isn't actually costing you anything compared to having it sit empty. Your agreement with the landlord was that you would continue to pay rent unless you found someone to take over the lease, which you didn't. Now, since your lease is still valid, you have the right to use the property, but you explicitly told the landlord that you didn't plan to use it for the next two months. The landlord could argue he isn't denying you as long as he would vacate if you changed your mind and wanted to move back in.
Regarding the security deposit, did you have your inspection or at least take photos at the time you moved out? That is the main potential cost to you that I see, if the landlord tries to charge you for "damage" that wasn't there when you moved out. But it's only at the end of the lease that he owes you an accounting/return of the security deposit, so in that regard he hasn't wronged you yet.
You might be in a better position if you had terminated the lease early. This would likely trigger an obligation of the landlord to look for a replacement tenant. Depending on local law, you might no longer owe rent after some period even if no new tenant was found. Likely that would be longer than two months, so you would still owe the remainder of your lease if no one was found. You could argue that by living there himself, the landlord has found a new "tenant" (imputed rent). However, even this might not completely relieve you of the rent, if the house is intended for more than one person and so the value to him of living there alone(?) does not make up for the rent in the lease.
EDIT: It looks like Florida is even more lenient on the landlord in this situation than most states. The landlord is allowed to "retake possession" of an abandoned unit while looking for a new tenant. And if he didn't retake possession but left it empty, he wouldn't have to look for a new tenant at all but could simply continue to collect rent from OP.
So if the landlord can argue that the property is abandoned, he may be within his rights to live there and charge rent as long as he is looking for a new tenant.