Your son would need to sign a note and a mortgage.
The mortgage would then need to be recorded.
That would put a lien on your son's property.
To secure a mortgage on his home you would need his consent.
You should also have a lien search run to find out what other liens are already on his property.
The title company or an attorney should be able to help you with this.
Just to follow up on the prior answer - if the son is not cooperative then you would need to file a breach of contract complaint and get a jugment against him and make sure the judgment is entered in ther county where the property is located or transferred to where the property is located
To add to the prior answers (just connecting dots they already provided), if you get a judgment lien, you generally cannot foreclose on his homestead. If the house is not his homestead, the judgment lien may be meaningful, depending on whether there is equity.
If he will give you a mortgage, and there is equity, that may be the way to go.
Please note that nothing herein is or may be relied on as legal advice. Each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship. Best wishes for a just and timely resolution.