sons teaching course..what visa

hello folks im looking for information on a few subject's probably should be in different forums bit all related to my son coming to Thailand do please excuse the  Mix first of all i live here permanent..my son wants to take a TELF course in Chaing Mai in October......last year he came for 3 months liked it so much wants to try to work here ..last year he got a 90 day visa from Paris my son holds both British and French passport ..its a lot easier to get a visa on his french ...But could he provide his British passport for applying for work..as i said October is the date of his course  and hopefully all this mess ref travel will be over But if its still a point that you must self confine on arrival as ive heard is the case would it have to be a place approved by the government ..would it have to be local to arrival point ie Bangkok or would he be allowed to travel to another area i have a condo at jomtien i rarely use could he travel to here ..as i said hope all is good by then  if not we're all in s..t creek eh .. thanks for any help