If he has a charge over then if the amount he is entitled to is paid off then he should have to provide a discharge accordingly.
I'm sure the court would assist if he had what he was entitled to (is it backed up by a court order) and still refused to sign the discharge. The court may be able to intervene and get it signed.
If it's a private agreement then the terms of the agreement and charge need to be carefully checked by your solicitor.
Is your solicitor having issues with giving an undertaking to provide discharge documentation because of the issue causing problems with the sale?
Is there a separate document you are trying to get him to sign now before you sell or do you just mean the discharge document to be provided after the sale when he has been paid off?
Might also be worth having a word with the Land Registry to see if they can provide any further advice about what they might accept if he still refuses to sign.