depend on the visa. Have a look. If it says"multi entry"then you just leave thailand and return without problems. Of course now a days you have to apply for COE etc etc.
Christmas this year, 2020?? forget it.just forget it. Maybe you have heard about corona, two weeks quarantaine and lots of paperwork. Absolute impossible.
But if he stays everytime two months in thailand and he can apply during the last 30 days of that period it will not be possible to ask for an extension of stay. Only if he is lucky and can aoply 45 days before it will be possible. I do have the same problem !