Allowed to Require Lump Sum Payment for Escrow?

I have a customer whose escrow payment for taxes was estimated at time of closing because his purchase was a new construction. The tax bill was not actually what we thought and we sent back a check for roughly $2500. His bill this past year went up and now the escrow account is short about $3600 compared with where it needs to be. We have just started escrowing in the last two years on all loans. Do we have to give the customer the option to pay in a lump sum or make it up through his payments or can we require a lump sum payment now?