As said, an "unlocked" phone should accept any SIM, the SIM's number will be what the phone then operates on. Currently a UK SIM can be used in Switzerland and vice versa. What will happen after 29th March is unknown although your son would do well to get a firm explanation what his provider(s) think(s)/intend(s) after that date - this should be in writing one way or other. If no firm answer/commitment is forthcoming then assume roaming may/will no longer be free (i.e. no longer same charges as at "home", whichever applies).