Buy a plain band in the same metal first. Something much cheaper.
Wear it for a couple of weeks non-stop. To bed, in the shower, gardening... You'll give your body a chance to tell you what it needs or doesn't need before investing in an expensive and hard-to-resell custom mount. Yes, it'll add two weeks to your journey, but in the grand scheme of being confident and happy with the outcome two extra weeks is nothing!
It's also a better path for him - remaking a mount in an alternate metal because you turn out to be allergic would be extremely generous of him because it'd definitely be a financial loss. By testing this metal out beforehand you spare him potentially unnecessary expense as well.
I believe eweddingbands has some for under $200. Or etsy might be an option.