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In my twenties, I did this to someone. At the time, I thought it was because I didn't want to hurt my parents. But really it was because I was a cowardly ass and I didn't think the relationship was worth stirring up too much trouble about. I knew it would end before my relationship with my parents did. And, of course, it did.
My guess is that your boyfriend has his doubts about how permanent your relationship can be and is afraid to tell you. On some level, he agrees with his parents that this is not the right relationship for him. Ask him, and if there's hesitation, show him the door. He should feel lucky he found you, not fearful of discovery.