I'm not entirely sure but, if there isn't any discharge from the eyes or nose then I don't think there is a problem. Your lop could just be allergic to something in the air, or it could be dust, dirty living conditions, it's bedding, hay, etc. I've also heard in the past rabbits will sneeze when disgusted at a smell, but I'm not sure if that's true.
I can't tell you much about my experience other than that one of my rabbits has the occasional sneeze too, sometimes 5 times in a row.
Have you heard of Snuffles? Here's a good article explaining.
My Felix and Clementine sneeze every so often. No mucus, no watery eyes, and it tends to go back and forth depending on the weather/hay type/food type/etc. I don't think it's anything to be concerned about unless it's happening extremely often, and/or is accompanied by a runny nose/watery eyes.
just to be safe i would keep him away from your other rabbits. i would not worry about it. but if it does happen to be the snuffles that could be very bad.