Sarah, lease-purchase is the terminology which is best suited for those who cannot obtain financing or those unsure of their financial status currently. You and your son need to locate an owner who does not need the proceeds from a sale in order for this work. They are out there but take some research to find. Connect with an agent experienced in these transactions.
In the meanwhile contact a reputable/qualified lender and see if FHA, USDA or some other special financing may be applicable for your son.