Yes, he can sell it to you for $275.00. Do not use a real estate agent as they will want a 5% to 6% commission on the sale.
There typically is not a co-signer for a mortgage, like there is for, say, a car loan where if the signer defaults, it is the co-signers responsibility to pay. When you take out a mortgage, whomever's name is on the mortgage papers are responsible for the loan repayment. So, your father will still have the house debt on his record and credit report. In the event that he wanted to take out a loan of his own, his income to debt ratio may not work for him to borrow money elsewhere for things he wants to buy because as a signer on the mortgage, whether he pays it or you do, his borrowing power drastically is reduced.
And, I am not quite sure how he could sign on a mortgage to the same house he is selling in the first place. Perhaps refinancing in both of your names may be a better option. Your father may want to draw a Will, leaving his house to you, where you would then apply for a loan for the balance on his loan at a future date. But, again, you would have to qualify for that loan on your own.