First contact your divorce attorney since he is in violation of the decree. Then if you can pay the balance due now to stop the penalties and interest from accruing do so since you are also liable for the debt until it is paid. Then sue his backside for the bill you paid and any refunds that were grabbed to pay the debt he was to pay according to the court document.
Sadly the IRS will not get in the middle of this argument and what he did or did not do on a payment plan will not be discussed/shared with you ... the IRS is going to grab anything they can from either of you until the debt is paid and there is nothing they will do about it.