I found the only thing that worked was Leverage. Without that, offering something in return could work, depending on the severity of his symptoms. Would he agree to filing jointly if part of that money was paid out to him? Would you be ok with that?
It's also possible that the Ls have a collegial relationship, in which case his L might present it like it's his idea.
Whatever you do, try to not negotiate with yourself before you hear back. Wait until you find out before making any new offers.
I had a two year divorce an faced that dilemma, do I withhold enough for filing separately so I avoid penalties for not withholding enough or do I withhold less so there is a smaller refund. Not knowing how to get the spouse to file jointly if terrifying. The first time I was able to file electronically using the typical codes used that year for acknowledgement. (Yes, she claimed I forged her signature when there was no actual signature required, well, that year.) The next year it was changed to requiring a signature for future records. She refused to respond to my efforts for comment. I filed for the automatic extension. In October we happened to have a hearing and her lawyer and my lawyer finally addressed it. Their brilliant solution was that she would sign if I gave her half the withholding refund. Of course she had little earnings, so the refund was entirely from my withholding. Unfair but it sure made me wish I hadn't withheld so much.
That's how I faced the issue. I don't know if you can require him to sign jointly. I would expect any deal would involve him getting some of your refund as incentive. Sorry. Just don't volunteer to do so. And make sure you control the refund's return. You know you can comply with any agreement, he's unlikely to reciprocate similarly.
For the years I was in divorce court I decided it was safer to have my taxes done professionally. That way if she made any claims of fraud or forgery it wouldn't land in my lap.
