Temporary resizing of titanium rings - how?

My fiance and I are getting married in September. We're going to be getting titanium rings, for at least a while (we've agreed to get some gold rings later when we can afford it, as a personal preference). Titanium rings cannot be re-sized, but my fiance's fingers change in size quite a bit, from 11 to 13 depending on the time of year and his weight. We've decided to go ahead and get him a ring in size 13, since that's what he's wearing now, but we're getting into hiking later this year and are probably going to be losing some weight.

So, the question is, if he loses some weight and his fingers get smaller, how could we temporarily re-size his ring? We don't want to get another ring that's smaller, since that would kind of defeat the point.

Thanks for your help!