It does seem weird that your parents still haven't met your boyfriend, after this long. I've been with my boyfriend for nearly 8 months, and my parents had already met him a while before we started going out, because he was a friend of mine. But even if not, they'd have met him within the next few weeks, because we're always round at my house.
I never really had a big deal about the sleeping over at his situation. At my house he sleeps on the settee, and I sleep upstairs in my room. At his house, we both sleep on a double airbed, because he normally shares with his brother. When my parents first asked where I slept at his, I just said they got an airbed out for me, so they didn't know we were sharing a bed.
But since then, we've both started at university, and my parents know we stay over in each other's hall of residence, so are clearly sharing a bed. They don't seem to have any real objection to this.