He sounds angry, bitter and immature. I understand how difficult it is to try and ignore someone you once cared about and is now causing you pain, but that is the best approach. The people that really know you, those that you consider friends, aren't going to be influenced by this.
You can confide in others that you trust about the specifics, but please don't feel obligated to do so. It's really no one else's business. If you're confronted with questions, it's perfectly acceptable to simply respond that it ended badly and he's behaving childishly in not accepting that it's over.
Don't be baited by this. This defines who he is. Has nothing to do with who you are.
That was a hard but cheap lesson. Be VERY careful who you get that intimate with. I say that as a 40 year old man with one partner before my wife of 20 years. I regret giving my virginity to some girl I am embarrassed of now so protect all of you "firsts" and don't just spend them on morons. Guys at (presumably) teenage want to be that "I get plenty of girls and don't care about any of them" guy. Know that all the crap he is spewing is more about him than you and if you don't react he will quit.
That being said, if you wanted to own it I guarantee it would stop if you said "His was the first one I had ever seen so I didn't realize how small it was. That one was just pathetic". He will quit bringing you up in conversation pretty quick.
Are their photos or proof? If not, deny it. Also, people say these things to get a reaction out of you. If you do not react, then eventually they go away. Wishing you all the best.