Only if he's arrested and charged, with that nothing much will happen.
There do not need to be arrests or charges in Thailand, US authorities can instigate their own investigation unilaterally...
" Section 2423(c) of Title 18, United States Code, prohibits United States citizens or legal permanent residents from traveling from the United States to a foreign country, and while there, raping or sexually molesting a child or paying a child for sex. Citizens can be punished under this law even if the conduct they engaged in was legal in the country where it occurred".
That section obviously precludes the necessity for an arrest or charge existing outside the US.