If the relevant statute in GA is 6 years, that's 6 years from the date he stopped paying.
In any event, it's critical that you have a recorded trust-mortgage on the property. No one in their right mind would buy the property (and no average lender would loan money for a place) with an existing second mortgage. A recorded mortgage will prevent its sale (just as it won't be sold without the first mortgage being paid).
Of course, if the place isn't really worth as much as you think, it may be that only the first mortgage will be able to be satisfied. (But this doesn't mean you don't have time to sue on the promissory note.)
You need to discuss these issues with a local real estate attorney. If you didn't have one when you did this, that was a bad idea. If you did have an attorney and didn't come out of the transaction knowing you had a security interest in the property, I wouldn't be using that attorney again.