My son (23 yrs), Swiss citizen, is currently on a Non B Visa + working permit.
We (Thai mother) eventually managed to get him a Thai ID and successfully applied for Thai passport.
What needs to be done to cancel his current visa/let the authorities know that he is not in Thailand anymore based on his Swiss passport?
TLDR : Answer Summary
To cancel your son's Non B Visa after obtaining Thai citizenship, he should first fly out of Thailand using his Swiss passport. Upon exiting, he can then re-enter Thailand with his new Thai passport. Since he is on a 90-day Non B Visa without an extension, the visa will be automatically canceled upon departure. After returning on his Thai passport, he can proceed to the Ministry of Labor to cancel his work permit by presenting his work permit booklet alongside his Thai ID card.