Untaxed income transfered

If mother has no taxable income then she is not compelled file ITR. Any money given by mom to son is exempt from tax.

No impact on mother and son with respect to gift.

the son can claim deduction of interest under section 24.
but i am not sure about 80c deduction on principal relayment of homeloan. since it is not paid out of his own taxable income the deduction may not be available. you may consult an expert on this aspect.

It is exempt from tax