there’s no high horse or need for you to be melodramatic, also there is no “he’s legally allowed”, he can stay and enter as long as thailand immigration lets him
His son and family is irrelevant on tourist visas or visa exempts
afaik you don’t have to buy the ticket and it’s on the airline, maybe if they give you the option of going to a different country than your departure you would have to?
happened to me going to Vietnam last year when they were trying to hustle me for a new visa, i told them to put me on a plane back to bangkok