Advice?

My boyfriend and I have been together for almost three years. We're not engaged but we're getting together in June, we're literally together everywhere when I'm off of school and he doesn't work, which means mostly Friday night-Sunday, if he's going on a road trip with his family I'll miss school and tag along if I'm going somewhere with mine he'll miss work, doctors appointments, literally anywhere we can go together we'll be together! My problem right now is I still can't get my family to refer to us as one, this weekend my family has a road trip planned. We were having dinner last Saturday and my mom asked me 'are you going?' and he was there and I said 'yea, WE'RE going.' and he just kind of looked down. This has always been a problem and it's really starting to upset him, he's an amazing hardworking independent guy and I know my family likes him I just don't get why they can't start referring to us as ONE. His family always does, his mom or sister will call me and ask if we're going to parties with them or anything like that, they'll ask if I'm ok with certain changes in their house things like that. What could it be? I'm so confused and it's making me upset that he's getting upset about it.