He can continue the ESI contribution till the end of the Contribution period only. Refer this link for Contribution under ESI: https://www.esic.nic.in/contribution
An employee whose gross emoluments (excluding bonus, OT, leave encashment, gratuity) exceeds Rs.21,000 are not eligible to be covered under the ESI Act.
An employee whose salary enhancement/ appraisals results in exceeding the Rs.21,000 criteria will continue to pay till the next ESI contribution period. The two esi contribution periods are 1st April to 30th Sep and 1st Oct to 31st March.
For instance if an employee earning Rs.20,000 get an salary increase to Rs.22,000 in the month of May. He will continue contributing ESI on the revised salary upto Sep of that same year.