Well, this is supposed to be "his" cat and he should name it. YET, he didn't pick a name for either cat now did he? His ex picked the name and then YOU picked a name for this one so theoretically he hasn't really picked a name of his own. He SHOULD pick a name of his own, "not" a name you or his ex picked but instead his own. I don't think this is about his attachment to his ex either, I think it's more about how he has a habit of needing someone else to name the animals in his environment because that's probably what he experienced in his history where all animals were given names by someone else and not him and he just subconsciously accepted it. He did not NEED to pick a name himself either, and as I mentioned he didn't, instead he let YOU do it. It probably bothered him that you picked the same name as his ex and did not want to tell you that it does remind him of his ex.

I think you are putting so much of "your" emotions in this that you are not actually seeing that it's not what you "feel" it is.

Actually, the conversation you could have with him about this is more about how he should really pick his own name, not the name you or his ex happened to pick. You could say to him that it's better that he see how he unknowlingly let someone else pick for him when he should gain his own confidence in deciding things for himself instead. The name of this cat could be his reminder that it's his cat and he can pick the name and every time he calls out it's name it's a reminder of his independence and ownership and personal choice and "not" someone else's choice.