He is planning on selling his 50% of the home

There is currently nothing to stop him from transferring his interest to anyone else or to a company. You could file a Certificate of Pending Litigation against title (this requires you to commence a proceeding by way of a Notice of Family Claim) which would likely at least temporarily prevent any change in title). If he stops paying, you may be able to seek spousal support from him to help contribute to the mortgage expense – although it initially appears that you may be barred from claiming you are a spouse under the FLA (because it has been more than 2 years since you separated), however you could argue that the two years time limit was extended (or suspended from running) by reason of his continuing to voluntarily pay support to you by continuing to pay the mortgage for the past 5 years such that you had no reason to file for spousal support because of his agreement to continue to pay the mortgage. If you are found to be a spouse under the FLA then you can also claim a division of debts including the mortgage. Any new property owner would take their new interest subject to the prior mortgage on title such that if the home was sold, they would net 50% of the equity after the mortgage has been discharged from title and I believe that you would have a claim against the new owner for an accounting of half of any property taxes, property insurance and possibly any reduction in the principal owing on the mortgage by reason of your payments. You want need to speak to a Real Estate lawyer on this latter issue of an accounting to the new corporate owner and your rights if any to seek contributions to the mortgage from them.