Using subscriptions may be a good idea. This way, user will definitely know that it is just a subscription and it can be immediately invalidated when the time comes.
Another good idea might be to use a server. You can store all the data about the purchases and information about the users. Then you can check the purchase status whenever you like using the beloew Purchase Status API
https://developer.android.com/google/play/billing/gp-purchase-status-api.html#overview and then you can disable users purchase status if the purchase was 2 months old.