The thing is, that every time you get a boyfriend or thinking about getting one. The family looks at this guy/girl as a potential family member. They don't think of them as someone you will spend time with and move on. If you want to break the ice just tell them, we just like each other, there are no wedding plans being discussed, can you just trust me on this one?
You need to find out the reason why your family members aren't getting along with him. If it is something not too serious to warrant such a treatment, you can talk to both parties with the aim of ironing out the existing differences. I believe you can play a role in shaping out this relationship such that it is both agreeable and successful.