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Is your son's SS death benefit payable to you on behalf of your son? If so, and it will continue for the first 3 years of the mortgage, then you can use it to qualify. If it's not payable to you on behalf of your son, or it won't continue for the first 3 years of the mortgage, then it couldn't be used to qualify. For example, I had a client whose son was receiving social security survivors income, but it was in the non-borrowing spouse's name. We had to get it switched into our borrower's name + provide a copy of the "Benefit Verification Letter" + provide a copy of the child's birth certificate to prove it would continue for another 3 years (until the child was 18).
Your loan approval may require you to have a certain amount of money in reserves, which is why you've been advised to not pay off your credit cards, because then you wouldn't have the money you used to pay off the credit cards with to qualify. Ask your loan officer for specifics on why you can't pay down/off your credit cards. It could be that paying off the credit cards won't help your DTI any.
Do you need your son's social security income to qualify? If he just turned 15 in August, and it'd end when he turns 18, then it doesn't sound like it'll be received for the first 3 years of the mortgage.