Sec 54

First:  Transfer of property by father to son.

The transer is exempted u/s 47 there will no capital gain in the hands of father.

And no tax in son's hands as it is a gift and gift form relative (father) is not a gift u/s 57. Hence not taxable

Second:  Sale of property.

On sale of property by son, the capital gain will be taxable in the hands of son, as no clubbing provisions will be appicable on this situation, till the transfer is irrevocalbe.

Third: Claim Exemption

To claim exemption the house must be purchased, 1 year before the sale or 2 years after the exemption or constructed within 3 years of sale.

But in your case the house is purchsed in september 2010 , which not fulfilled the both condition of purchasing the house i.e. 1 year or 2 year.

Purchase need not necessarily be on ‘cash and carry’ basis - The word ‘purchase’ in section 54 must be interpreted in its ordinary meaning, as buying for a price or equivalent of price by payment in kind or adjustment towards an old debt or for other monetary consideration. There is no stress in the section on ‘cash and carry’. Thus, where the eldest brother in a coparcenary comprising four brothers sold his own house and acquired the common house from his three brothers who executed release deeds for a consideration, there was a ‘purchase’ by the eldest brother of the share of each of the brothers for a price - CIT v. T.N. Aravinda Reddy [1979] 123 Taxman 290/120 ITR 46 (SC).

Please emphasize on the word written in bold. so If cap gain is utilised for setteling the loan of another house (i.e old debt) it will tantamount to Purchase in the view of the Act      

Under Sec 54 of the IT Act,1961:

1) Transfer of Capital assets by father to his son is not transfer (Sec. 47) Hence not taxable.

2) For claiming exemption : Purchase new house 1 year before or 2 years after the transfer OR in construction case, house must be complete within 3 years.