What do I do as a wife who loves her family,but one overbearring 16 year old stepson is hard to deal with?

My husbands oldest son wanted to come and live with us for a year. We both agreed that it was a good idea. Especially for him and his son to get to know each other and make up for the lost years. Everything was fine for a while, but when some boundaries were set and rules for him things dramatically changed. I try to stay neutral and be fair. It is hard for me when I can see how stressed my husband is getting. I know it is going to take them both to make this relationship work, but so far it's not working.