I had this funny thought float into my head...
"It's OK for you to be in a band, but only if I get to sing backup."
It's OK for you to tell him that this condition will not work for you. It's fair that he doesn't understand what polyamory means, and it's a reality that it will fall upon you to explain what it means for you.
For you, this is not about sex. It's about love and intimacy and an emotional connection. By the sound of it, he doesn't even have the time to properly maintain one romantic relationship. Who does he think he's kidding that he can take care of two women??
Five years in, I'm willing to bet he'll be willing to at least consider other possibilities if the success of your relationship depends on them. Give him time, be ready to answer questions, and stay true to your needs.
And, of course, on the other end, opening the relationship means that he can also have sexual relationships with other women, as long as they practice safe sex in a way you're comfortable with. That should be music to his ears while he's gearing up for going on tour...
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker