PPF investment in minor name

Yes.

The PPF accounts of minor/s are open under guardianship, which means any person cannot invest more than 1.50 lakhs in aggregate, per annum in his own or minor's PPF account/s.

If at all its accepted by mistake, it is paid back without interest whenever the mistake gets recognized.

1. Investment in PPF can be made on behalf of minor to the extent of Rs. 1,50,000 per year. One person (including minor) can have one PPF account each. 
2. In your case son's account cannot have an investment in excess of Rs. 1,50,000 per year. Whether it is made by father or by mother. The aggregate amount cannot exceed Rs. 1,50,000. 
3. The claim made by HDFC is valid if investment in Son's PPF a/c has touched the threshold limit of Rs. 1,50,000 during the year. 
Please correct me if the above solution has an alternative view.

Yes. That can be done.