Why wont my boyfriend introduce me to his family?

I am 20 and he is 31. The relationship has moved quite quickly as we have been seeing each other for 6 months and now live together. He wants to get serious as there has been talk of marriage and children but whenever I bring up his family he just rolls his eyes. He has met my mum but i dont want to introduce him to people im close to because I find it wierd that he is so against me meeting his family.He says he isnt close to his family yet he sees his brother every week and his mum every other week, I think its a natural progression to introduce someone to your family. He hasnt told them we are engaged and I wonder do they even know about me.