This is completely understandable. If you are not comfortable with telling your parents, then don’t tell them. You need to be comfortable as well, especially if the relationship hasn’t gotten serious yet.
I think he is worried that you are trying to keep him a secret, and I am sorry if I am wrong. I wouldn’t keep it a secret, but I would wait to announce a serious relationship until the relationship has progressed a little and you have had some deep conversations about the specifics of what the relationship means, just like any other relationship.
Let him know that you will tell them in time, but you don’t want to rush in to anything, that you are not comfortable telling them yet.
Other than the relationship needing to progress, is there anything else that may be holding you back? Are you worried about how your parents may react (even though they know everything else), because of how your sisters reacted? Has your relationship gone public and the only people that don’t know are your parents? All of these things can contribute to your dilemma.
If you would like to talk more please feel free to ask, or come off anon and we can talk privately. Also if anyone has any tips or stories they would like to share please feel free to submit them and get some interaction going!
I hope everyone has a great week!