The penalties may be imposed on either the Plan Administrator or the Employer. (Usually one and the same for small plans, but not always). You can strike fear into him by quoting the penalty amount. And don't forget that the IRS can impose penalties as well. Also, was the plan amended as required (or restated) for the termination? Yet another landmine.
Probably goes without saying that you should get paid up front before doing any work...
Note the following from the DOL FAQ's on the DFVCP program:
Q15. Can plan assets be used to pay the civil penalties assessed under ERISA § 502(c)(2)? No. The plan administrator is personally liable for the payment of civil penalties assessed under ERISA § 502(c)(2). Civil penalties, including penalties paid under the DFVCP, cannot be paid from the assets of an employee benefit plan.
Willful failure to file can result in CRIMINAL penalties on top of all the other fun.
I'll bet you can scare him sufficiently, but if not, tell him to enjoy bankruptcy and possible criminal charges. Playing Russian Roulette with the DOL is an unrewarding form of entertainment. (I know I'm preaching to the choir on this.)