So what concerns me here is this. If he is Caucasian, than it seems real unlikely he has specifically a fetish for white girls. It seems like best case scenario he has some subconscious bias, and worst case scenario he is kinda just racist.

Do you have any interest in a threesome? If the only reason you would consider that is because you are afraid of losing him, Don't do it. It will not solve the problem, and will only make your feelings of inadequacy far, far worse, by manifesting them in a real life flesh and blood person that you now have to compare yourself to during sex.

But the bottom line is that it’s something he is curious about and will act on if he has the chance

Not thrilled about that either. I know you said "with my permission" and all that, but what happens if you say no? Does he keep finding new girls till one sticks? Won't you feel pressured/obligated to approve?

Let me be clear, it's okay to have a preferred type, as far as physical attraction goes. But that's not what's going on here. He's saying he can't be fully sexually satisfied/fulfilled by an Asian, or only an Asian girl. Usually when people have "a type" but end up fating outside that and falling in love, they don't need to still go and find someone of that type. So many people will tell you that their significant other/spouse wasn't their typical type, but they fell in love so it doesn't matter, and they are just wholly attracted to the person they have that bond with.

Honestly, I think you need to find someone who respects you completely, and isn't probably low key a little bit racist :/ Maybe that's not an ideal/practical solution for you right now, but at the very least, don't get pressure into a poly relationship/threesome unless the thought of that turns you on as well.