A newborn, Illnesses and a stubborn stepdad

Right now my husband and I are living with my mom and step dad and we're expecting our first baby any day now. I just got into an argument with my step dad because he says you have to expose your babies to germs to build their immune system. I told him I'm not going to expose my newborn to illnesses and risk his life just to build his immunity. He said he's had 7 children and he knows what he's talking about. First of all, his youngest is 27 things are a lot different now. Second of all he will argue with a fence post. I love him very much, don't get me wrong, but he thinks he knows everything. I've gotten into it with him when he tried to feed my dog onions! I told him dogs can't have onions among many other things and he argued with me. Their dog is obese and sick, just saying. We don't have the money to get our own place yet so moving out to keep the peace isn't an option. He's so stubborn and hard headed, I tend to just stay in my room all day until my husband gets home from work because I feel like if I say anything more than "Good morning" he'll find something to argue about! What can I do to let him know that with my baby and my animals, I make the rules whether we're living under his roof or not, and not have it turn into an argument?