PPF investment in minor name

1. I make certain investments in my name, some in my wife's name, and some in our kids' names.
2. I club the return from all these investments (Section 64) and pay the required income tax.
3. The investments include 1.5 lakhs each in my own name, my wife's name, and our major daughter's name.
4. I periodically give my wife money for her personal expenses which are credited into an SB account. There is no other source of income for her.
5. From this money she wishes to invest in PPF account of our minor son.
6. All 4 of us have separate PAN.

Now HDFC Bank refuses this investment in our son's name, stating that a person cannot invest more than 1.5 lakhs per year. Given the above facts, is this valid?

Thanks.