Welcome and congratulations on your engagement!
I think many bees can relate to your issue. Once you yourself are convinced, how do you convince the person who will actually be giving the ring to you, especially when they feel like they have a whole load of pressure on them from others?
I think it takes time, so definitely give him that. Obviously it’s important that he knows that you genuinely love this stone, and not just because it is the cheaper option. As you know, moissanite has many other merits other than cost, so perhaps you can start discussing these.
I understand fearing that he will look cheap, especially as the man buying the ring, but really anyone close enough to you to know it’s moissanite should also understand and appreciate your reasoning. If you’re anything like us, you may want to focus on some of the following points. Both my Fiance and I try to make the most ethical decisions in life wherever possible, so the fact that the stone is not strip mined was a big one for us (the environmental issues were a concern here, not just the political/humanitarian). I don’t place much emphasis on a stone being natural either, but I know others do (and that’s fine, of course). It also has a higher refractive index, which means it shows more flashes of colour. Some people like the white flashes of diamonds (and so do I), but I LOVE colourful sparkles, so here again moissanite was perfect for me. The cost, of course, is also important. We were prepared to part with a large amount of money, but with moissanite we found a stone that suited us better for the above reasons AND allowed us to use the extra money elsewhere. That’s not a bad reason either. Some people accept the high price of diamonds and others question whether it really reflects their worth. To me there is no right or wrong here because everyone puts different values on different things that are important to them, but here is an (old but) interesting article that may help your Fiance understand the diamond industry a little better.
Other reasons people can be wary about moissanite are due to concerns about it holding up as an heirloom and having lower resale value. As the closest contender in hardness to a diamond, there’s no reason why a moissanite ring wouldn’t last just as long. The resale loss as a percentage of total cost I’d say is true, but unfortunately most people find that reselling a diamond loses them more money in actual dollars.
I think sometimes it comes down to the sentiment that is attached to diamonds. Sometimes people can change the way they feel, but there’s not necessarily a right or wrong – it’s like convincing someone who loves living in the countryside that city living is better. But ultimately, you should be choosing first and foremost based on what makes the two of you happiest, not what others will think. Your choice won’t affect anyone else but the two of you. Good luck!