I liked his post about grad school... he still blames me

My boyfriend and I have been dating for 9 months now. Early in the relationship around 3 months, I liked a guy's (a ONS) post on Fb saying that he got into grad school. It was just to say congratulations because I know grad school is difficult to get into. My boyfriend is now furious that I did that saying things like 'How could you sleep with someone that you had no connection with' 'How could you support someone who was mean to you' and 'I would never do that because I have morals that you're lacking. ' Everytime that I tried to explain that I didn't mean any harm, I'm not looking to connect with him, I don't want him in my life, etc, my bf immediately shoots me down and continues to blame me. Right away I deleted the guy from Fb and anything associated with him without any trouble because I don't want him in my life. Bf is still mad and won't let it go. After declaring that that's only happened once, promising that it will never happen again, and apologizing numerous times, he still holds it over me. I've tried everything and an just not sure what to do anymore. Any constructive suggestions would be appreciated. Thanks