|Hi Sandy -
Your son's rash is not contagious. Once he's been washed along with any contaminated clothes, toys, etc. there is nothing for him to spread. Your pets should be washed to make sure that they don't have urushiol oil in the fur. If they are not out or not likely to run into poison ivy, then there is no need for additional washings. As for your son's sheets, I'd wash them as often as needed. The blisters can weep a lot and crusty sheets are no fun to sleep in. It's more of a desire to be in clean and soft sheets as there is nothing for him to spread.
If your son sat on the furniture and spread the urushiol oil to the furniture, then yes there is a slim chance the rest of the family could end up with a rash. Think about where he was and how he could have become contaminated. Those items need cleaning and once should be enough.