Under Thai Law, his employment status become open from 1 April 2014.
His contract was made under Thaiwah Plaza (TWPL) started 1 April 2012, so his year of service with TWPL is 3 years and 10 months. He will be entitled to severance equal to his last 6 months' salary + fix allowance plus 3 months advance notice paid in lieu thereof according to his contract.
My view on commencing date; his commencing date with BTHR Group is separately. His contract seems considering that BTHR second him to TWPL for 2 years but not formally inform on renewal.