When closing construction perm loans, most banks give a payment shock letter to the borrower at closing. It tells them that their escrows are being set up based on the current land only taxes and that they should expect a significant jump when the property is reassessed. If you did that, then there are no issues with the increase. Even if you didn't give the letter, the borrower had to have some inkling that their taxes would go up because they built on the land. imo, there's no obligation to give any monetary concessions.
I'm at a loss as to why your escrow analysis letter gives such a short time frame to pay the shortage. I suggest you dig into that issue internally while you do what Randy suggested and offer the borrower a more reasonable repayment schedule.