He will have to dump it at some point. You cannot sit on property that has a loan that is outstanding for more than the property is worth. (Unless he has a lot of extra disposable income. But if that was the case, he would not of taken the loan out in the first place.)

Did you offer cash for the property? (if you can afford it). Cash speaks VERY loudly in many cases.

See if you can wait him out.

Keep trying, you have nothing to lose.