TDS will be calculated from 1st April to 31st March of every year. If he joined in the month of Feb'18 then you need to calculate Feb & March salary for this year IT and balance 10 months will be calculated in next year. If you are paying bonus after one year, then you need to consider bonus amount in next year as a his earnings.
TDS will be calculated considering other provisions of the act. Example- HRA, 80 C savings, Medical insurance, Housing loan etc.
19th February 2018 From India, Bangalore