In May-17, He gets OT of Rs.2500/-, Now Salary is Rs. 19,000/-+Rs. 2500/- = Rs. 21,500/- and ESI would be deducted on Rs. 21,500/-.(Not on Rs. 19K or 21K)
In September-17, His Salary is increased to Rs. 23,000/-. (Assuming no change in limit) From the next contribution period, He won't be member of ESIC, But in current contribution period ESI would be kept deducted. Thanks.