Excess TDS on salary

Yes he can ask his company to deduct higher TDS. Further, every company asks for declaration of any other income from its employees for TDS deduction, its better to declare the other income at that time. 

You need to declare in Form 12BA

Yes, one can ask the employer to deduct additional TDS on the basis of the information provided of the additional income.

Just keep in mind that this may not be information one necessarily wants to share with the employer, so think before disclosing!