firstly u have to check if ur son's income is eligible for TDS deduction....u can submit form 15G or H ( check it out) if he is not elegible to pay taxes...and the bank wont deduct ur sons TDS
if his TDS is deducted, he can claim deduction as he has interest income from the bank...and u can use it when u are clubbing his income with yours.
i think the 1st step is better.. submit 15G and they wont deduct TDS