If a person working with more than one employer in any previous year, then in such case he needs to produce either Form 16 or 12 B as per income tax act, so that the current employer can calculate the correct tax payable by the employee based on the information provided.
If the previous employer is unable to provide or not willing to provide the Form 16 then you can furnish form 12 B yourself.
Form 12 B contains the details of your salary, which you can exract from your salary slip of previous employer. Also in Remarks column of Form 12B, you can mention the income from other sources.
But ultimetly who is the responsible for the tax, it is you only.
So you need to make your employer understand, if they do not believe on form 12B or Form 16, then you can give a declaration letter that you have earned this much amount in that particular financial year.
They can't assumed your current salary for the whole year.
If still they are not willing to accept your point, ask to produce the evidence where it has been written that current salary will be presumed for the whole year as income from salary.