Ok OP, for some reason your dad convinced you that he "NEEDS" that card to build his credit.
Please answer us this. Is it a joint account or is it solely in your name, with him as an authorized user? If you don't know, seriously, you need to find out. This is basic information you should know.
If he's an authorized user, there is a very simple solution. Extremely simple. In fact, I think I mentioned it way back in the thread, but regardless. Call up the credit card company. Report both cards as being lost. Have both replacements sent to you. Cut up his card into tiny pieces, perhaps an industrial shredder? Then burn it. Then bury it (or dispose of properly according to your local regulations blah blah blah). Tell him you had the company replace the card, you're keeping it, and this will keep him on the account to rebuild his credit.
This will actually help both of your credit scores. The more he charges, the higher the utilization on the card. That does not look good.
Do this BEFORE you talk to him. Before, not after.
Now.
Not later.
NOW!!!
Why? He's given you a reason to keep the card open, to "help" his credit. He doesn't need it for emergencies, just to rebuild credit. This way, he's still on your card, just as before. His credit will still be "helped". He has his own card to manage for emergencies.
Jump NOW!!!!! at this golden opportunity. If you talk to him first about this plan, then there will be another reason given. Any plan you come up with will be countered. Simply cancel the card number (not the account, just that particular card number). Tell him after the fact. He'll then give you the "but you should really..." spiel. Tell him you were going by what he told you. You thought of a perfect solution to make everyone happy. If he keeps it up, just say this:
"You know, I'm beginning to really resent this credit card and the stress it's putting on our relationship. I'm just going to cancel the whole thing! I'm so angry at them for doing this to us!!!"
Yeah, it's not the card company's fault. But (mis)placing the blame on them might make your dad feel like this isn't an attack on him. If you're lucky, the reaction you'll get from him will be "No no, don't cancel it. We'll talk about this later." Then he can have a "win" by letting you do what you already did (cancel the current card number, get new cards issued that you keep, he's still an authorized user, his credit is not impacted).