Well, if he set up his own account, why couldn't he open a joint account?
You're not required to merge all funds per se, but for many it makes sense combining at least some funds for recurring costs such as rent, utility bills, car loans. From outsider's view (like USCIS adjudicator) it may look like one of you or both of you have trust issues in relationship manifesting in unwillingness to share finances. Like one YouTube lawyer said, sending money back and forth from individual accounts looks like roommates sharing bills? How are you planning to receive tax refund, for example, who will receive it? If you have a joint account, it just goes there.
I'm not trying to accuse you of anything, but laying out thinking an officer with old school thinking may have.
Absolutely print and add any other evidence of comingling life lile health or car insurance. Finance is a big piece of puzzle though and you may need to explain your arrangement in rational way to officer