First of all he should not be working under any circumstances if on visit visa (Father Sponsored). Companies usually grant a 3 month business visa during the probation period and then change to residence visa. In your case I am afraid you can not take any legal action as this is not allowed to work under visit visa policy. However, take closes look on the contract regarding the termination or resignation during the probation period usually it is allowed for both parties to decide to continue or terminate the contract.