U/s 54 of the Income Tax Act, "the assessee has, within a period of one year before or two years after the date on which the transfer took place, purchased or has within a period of three years after that constructed one ..." Thus the assessee has to purchase or construct... This is possible only if he the assessee is the owner or a co-owner of the property.
Now if he is a co-owner, you will have a problem in taking loan, as he is more than 70 years old. This is a catch 22 situation.
In my view, you can try these two following alternative courses of action:
1) In the case of a "purchase of a new apartment", you can perhaps add him as a beneficial owner in the sale deed, specifically declaring that he is a beneficial owner to the extent of his investment of capital gains in the apartment being purchased and he will be a consenting party to the sale deed, so that bank will not have any problem in lending, as he is not the joint owner. However, the property value for the purpose of the loan will be reduced to the extent of beneficial ownership held by your father. Thus you will be able to raise loan as he is not the joint owner and he is only a consenting party. From the tax point of view, he "purchased" his beneficial ownership, though not as a co-owner.
In this method, you will not be able to get loan on the full value of the property.
2) Your father can be still a joint owner but declare in the sale deed that his share of the property will revert to his children in whatever proportion he wanted to get it done. Thus the beneficial ownership of his children will get recorded in the sale deed and on this basis you may convince your bank to include the beneficial owners as the co-borrowers.
In this method, you will able to get loan on the full value of the property.
You need to draft the sale deed suitably in consultation with your chartered accountant and a lawyer, preferably the bank lawyer himself/herself.
Alternatively, if you can wait for 3 years, invest in Capital Gains bonds upto Rs 50 Lakhs u/s 54EC and in specified units u/s 54EE for another Rs 50 Lakhs. After 3 years, the money will be available to you without any restriction. This will be simple and straight forward.