What steps should my son take to cancel his Non B Visa after obtaining Thai citizenship?

Okay the O/P has only the initial 90 day Non-B visa, (not an extension) so nothing needs to be cancelled in that regard at immigrations (either before or after they fly out and back to switch nationalities..

Just stamping out on the 90 day admitted to stay stamp of a Non-Imm visa cancels it

Once they're back on the thai passport they can take the Swiss passport, the work permit booklet and their thai passport i/d card to the Ministry of Labor and turn in their work permit booklet..

Good luck to the O/P's son on the nationality switch (y)

In hind sight, I think he could just FIRST fly out on the Swiss passport, and back in on the thai one THEN go to the MOL to cancel the work permit because showing the thai passport and i/d card is enough of a reason for them to cancel it.

FLY out of the country on his swiss passport, then fly back IN to the country on his thai passport.

There is no other way to "switch to a thai" inside the country and you can't do it bouncing at land borders.

BUT

because this isn't a visa question but a how to switch nationality question I'm turning off commenting after providing the answer.

@Stephane ****

if you have questions you can message me