I'm not sure where you get the 90 day right to rescind a sale of a business. Your sale is governed by a contract and any advertising or other representations made by the seller. Can you provide more info?
Bankruptcy is a highly technical area of the law, and it's not recommended that you represent yourself. The bankruptcy trustee's job is to "marshal the assets" of the debtor in bankruptcy, which may include payment to you. Find an experienced bankruptcy attorney.
In some states, such as AZ, if you pay another person's property tax for so many years, you have a right to the property after so many years. If the property owner comes and pays the tax bill at the last minute, the tax payer receives his or her money back plus statutory interest. Depending on the economy, this could be a good return on your money.
Call the tax assessor in the area and inquire.
You've raised a good issue, but if I were to respond, then you would be communicating legal advice to your son. It's important to understand the attorney-client relationship, which exists for good reasons. One of the reasons is to ensure direct and clear communication between the client and the attorney. This gets distorted when third parties - no matter what the relationship or good intent - get involved. I appreciate your concern, but the best advice anyone could give you here is to have...