If he doesn't agree to have his name removed, no one will buy it, it is worthless to the Thai owners both present and future. He cannot sell it without consent. Without his consent to be removed, it is useless to the current owner. 2 edged sword.
The current Thai on the land title will need to agree to transfer it to another, whether 100%, 50%, 10%, 1% or 0% of the proceeds goes to him is between himself and that Thai has nothing to do with the land office or the person buying it.