His living with you will not be "held against" you. What matters is if you're capable of working any job over the SGA level. Your daily activities help to show the severity of your disability. Do you claim your grandson on your taxes? Do you use Your home address as your grandson's legal mailing address? If you answer no to both those questions, then he does not live with you. You can't hide your care of him, but what do you do for him? Do you do all his laundry, cook all his meals, drive him to school, etc.? Does your spouse contribute to his and your care? Or is he just sleeping in your home?

I think this is one of those occasions where you're worrying about something that is likely a very minor issue. Have you filed an application yet?