Does he owe us [a prorated refund]?

Probably not. Even if we consider you voluntarily leaving as "breaking the lease" (which it almost certainly was not as you had no legally valid reason to do so), the landlord only needs to make reasonable efforts to find new tenants, which he did before you even moved out apparently (and with your help nonetheless).

Are landlords allowed to "double dip"? In many circumstances, no. However, you essentially vacated your tenancy voluntarily without any mutual understanding as to whether your rent would be prorated. While you were entitled to stay for the rest of the month, the landlord was not required to sit on an empty unit or pay you back for the rest of the month. Essentially, you fulfilled the terms of your lease and decided to vacate early (and presumably, to move into another residence). Finding new tenants absolved you of your legal obligation to pay further rent, but it did not entitle you to reduced rent. You have no legal basis for demanding any refunds.

Now, from a more practical and more important standpoint, calculate what your prorated rent would be compared to half of the security deposit that was returned. Receiving your full security deposit is almost unheard of to me. Perhaps this was your landlord's way of giving thanks for your cooperation and early move-out? Even further, you state that this not about using the legal system--good thing, because you better include those costs into deciding whether or not it is worth it to pursue a refund. (Granted, you'd likely be dealing with Small Claims Court, which does not require an attorney and has lower fees.)