Let me put this into an analogy you may be able to understand. About “giving into his insecurities”. 

Before sex, every time he tells you “I wish you were as tight as my ex, but it’s ok. It’s in the past. It doesn’t matter that you don’t feel the same. It’s in the past.”

The problem here is the one tattoo. The snake. 1:1 representation of your ex’s anaconda is a daily visual reminder of your ex and his “endowment” as you’ve put it. 

Go ahead and be pissed about it now, but now that the door is open, he’s curious. The snake just makes him wonder about the other. 

At the bare minimum, ditch the boa constrictor. We get it, your ex was huge. Your current doesn’t want a daily reminder.

Edit - basically it’s your body. Keep it if you want, but be prepared to lose him.

Edit - actually second thought. Honestly this may already be too much for him. Therapy may be needed, if not definitely open comms. 100% open and that may lead to things that can’t be undone. BUT, with this don’t ask don’t tell thing being blown, it sounds like he found out more than he was prepared for.