It is mandatory on part of previous employer to furnish form 16 to you for filing IT return but why your present employer is bothered about from 16 of previous employer.Infact they should give you form no 16 for the period for which you have worked with them.
They should take a declaration from you for the previous income & investment made by you if any & calculate tax acordingly.It will be your responsibility to file IT return taking income of both the employer during the financial year.
You can also draw your previous form 16 from the salary slips which you must have received on monthly basis for filing IT return clubbing with the present one.