Never alone to have sex?

My boyfriend and I are both going off to college, he's a sophomore and I'm a freshman. We've been dating for 2 years, and I finally felt comfortable enough to have sex with him. We were both virgins, everything went swimmingly, wahoo!

Issue is, the firs time we "did it" we were incredibly lucky, his step brother just left the house and right as we were done his mom was entering the house and I hid in the bathroom. The second time it happened his step brother literally just got home as again we finished and I was getting my clothes back on.

So the issue here is we want to get more experience but the house never seems empty for long enough (or it is literally for JUST long enough). His parents are never home in the day until 5 but his step brother is always home and doesn't leave for work until 4. It leaves an hour but it's usually a hit or miss if his mom comes home early or something interrupts. My house is off limits because my parents are always home or unpredictable.

So is there anything we can do? I don't know if I'm open to "public" sex (really anything off a bed.... Maybe couch. Or are we just on our own until we got back to college (we are at close schools) since he has a single room.. Thanks for the help in advance!