That guy will spend the rest of his life in prison or he might be executed so he confess so he could have the peace of mind before he leave this world . There is no reason for him to lie.
Watch this clip..Guys.. You can see how dirty Old man Suthep was telling a liar about popcorn body guard. I'm not thai but I understand and could tell what is right or wrong.
And Most thai person are not stupid. They watched from Youtube.