How can I help my boyfriend understand it's okay for my salary to be higher than his?

We, not me. In forming a family unit, the emphasis is to create "we", not "me". One's salary does not become "mine", but "ours". There is truth to the cultural concept in marriage when two become one. It takes maturity, wisdom, and time to foster a healthy relationships.
Salary. I get feeling that u both are worried about this, as i remember a boyfriend asking the same question about his gf making more than him. In my opinion, if u both are happy with what u do, and care about each other, all the rest can be trivial, as long as u both share responsibilities financially.