I don't know what to do about relieving the pain for your son, but have you tried letting him drink everything with a straw? He will get smaller amounts at a time, and the straw helps direct the liquid better, so if his sores are more in his cheeks, it will minimize the contact, and therefore the pain. Don't worry about whether he is getting ALL of his nutrition over the next few days. He will more than make up for what he isn't eating now, once the sores are healed!!
I agree with the other post about your new baby getting it, too. If it's possible, try to minimize the direct contact between your children for a few days, though, just to be on the safe side. You might want to call an adult friend to come help you, too.
I'm feeling for ya, and praying for you and your kids!