Parents are allowing son to use their trust as collateral to purchase a home.
You are going to have to give more details such as is the trust purchasing the home or is the son purchasing the home using the trust's assets? What assets of the trust are securing the loan?
Also, I would strongly suggest you have an attorney review the trust agreement. I find it difficult to believe that a trust agreement would allow the trust's assets to be used where the transaction does not benefit the trust and this transaction benefits the son, not the trust.
The opinions expressed are mine and they are not to be taken as legal advice.